docs: scaffold docs/.style for the prose style guide (#25466)

Adds a private contributor-tooling directory at `docs/.style/` that will
host the canonical prose style guide and the custom Vale rules used to
enforce it. The directory's contents do not deploy to `coder.com/docs`.

This PR is the scaffold only. The Vale configuration, the rule set, and
the per-rule style-guide sections all land in follow-up PRs.

## What changes

- New `docs/.style/` directory with:
  - `README.md` explaining the convention
  - `style-guide.md` as a table-of-contents scaffold
- `styles/Coder/README.md` placeholder so Git tracks the empty Vale
rules dir
- `.github/workflows/deploy-docs.yaml`: skip the workflow on
`.style`-only pushes, and exclude `.style` paths from the
surgical-reindex git diff on mixed commits. Defense-in-depth on top of
the manifest-driven coder.com routing.
- `.github/.linkspector.yml`: add `docs/.style` to `excludedDirs`
- `AGENTS.md` and `.claude/docs/DOCS_STYLE_GUIDE.md`: cross-link to the
new style guide for agents

## Verification

- `make pre-commit-light` clean (`fmt/markdown`, `lint/markdown`,
`lint/typos`, `lint/emdash`, `lint/actions/actionlint`,
`lint/shellcheck`).
- `markdown-table-formatter --check` and `markdownlint-cli2` both
process the new files (existing globs are `find docs -name '*.md'`).
- `actionlint` clean on the modified workflow.
- coder.com exclusion works because route discovery and Algolia indexing
are manifest-driven; this directory is not in `docs/manifest.json`. The
workflow changes are defense in depth.

<details>
<summary>Implementation plan and decision log</summary>

### Decisions

- **Location**: `docs/.style/` (leading dot, mirrors `.github/`,
`.vscode/`, `.claude/`). Vale's `StylesPath` will be
`docs/.style/styles/`; `.vale.ini` lands at repo root in a follow-up.
- **Existing public page `docs/about/contributing/documentation.md`**:
untouched in this PR. Nick's separate information-architecture rework
will redirect it to GitHub at the right time.
- **Placeholder for empty `styles/Coder/`**: real `README.md`, not
`.gitkeep`. Discoverable on GitHub, lints with the existing tooling,
lists the planned starter rules.
- **CONTRIBUTING.md**: not touched. It's a 2-line redirect to
`coder.com/docs/CONTRIBUTING`; bloating it would defeat the redirect.
- **`.claude/docs/DOCS_STYLE_GUIDE.md`**: kept as the structure/research
companion. A blockquote at the top points at the new canonical prose
guide.

### coder.com exclusion mechanism (verified by inspection)

Direct inspection of `coder/coder.com`:

- Route discovery in
[`src/utils/docs/docs.ts`](https://github.com/coder/coder.com/blob/master/src/utils/docs/docs.ts)
iterates `routes` from `docs/manifest.json`. Files not in the manifest
never become routes.
- The Algolia surgical indexer at
[`src/utils/algoliaDocs/surgical.ts`](https://github.com/coder/coder.com/blob/master/src/utils/algoliaDocs/surgical.ts)
explicitly skips paths not in the manifest, incrementing `pathsSkipped`.

Net result: not adding anything from `docs/.style/` to `manifest.json`
is the only thing that has to be true for the exclusion to work. The
`deploy-docs.yaml` tweaks are defense in depth.

### deploy-docs.yaml changes (pre-mortem)

1. Trigger path negation `!docs/.style/**` skips the workflow on
`.style`-only pushes. GitHub Actions only suppresses when every changed
file matches a negation, so mixed commits still trigger.
2. The git-diff pathspec `:(exclude)docs/.style/**` drops `.style` paths
from the surgical-reindex payload on mixed commits.

Risks considered:

- **Test contract**: `.github/workflows/test-deploy-docs-diff.sh` only
exercises the downstream awk parser, not the git-diff invocation. The
exclusion happens at git-diff time; the parser sees the same
`<status>\0<path>\0` format. No test change needed.
- **First push to a brand-new branch**: the workflow falls back to
whole-branch reindex when `BEFORE_SHA` is all zeros. Whole-branch
reindex re-extracts records from the manifest, which still excludes
`.style` files because they are not in the manifest.
- **Workflow-dispatch**: takes the whole-branch path; same reasoning.
Safe.

### Why a real README in `styles/Coder/` instead of `.gitkeep`

It explains intent, lists the upcoming rules, and lints with the
existing tooling. The cost is one extra Markdown file; the upside is
that a contributor browsing GitHub sees the plan without clicking
around.

</details>

---

*Filed via [Coder Agents](https://coder.com/docs/ai-coder/agents) on
Nick's behalf.*


Linear: DOCS-180

.github/workflows/deploy-docs.yaml

@@ -45,7 +45,14 @@ on:
       # Intentionally only docs/**. Edits to this workflow file must not
       # auto-trigger a production reindex; use workflow_dispatch instead.
       # See DOCS-121 (incident) and DOCS-124 (fix).
+      #
+      # docs/.style/** is contributor tooling and never deploys to
+      # coder.com/docs. Negating it here skips the workflow on .style-only
+      # commits. GitHub Actions only suppresses when every changed file
+      # matches a negation, so mixed commits still trigger; the surgical
+      # diff step below drops .style paths from the payload.
       - "docs/**"
+      - "!docs/.style/**"
   release:
     # Fires when a draft release is published, when a release goes from
     # prerelease to non-prerelease, or when a release is created already
@@ -174,7 +181,11 @@ jobs:
           # + save) from deleted/renamed-old-side (delete only), and
           # so paths containing whitespace or quotes survive intact.
           DIFF_FILE=$(mktemp)
-          git diff --name-status -z "$BEFORE_SHA" "$AFTER_SHA" -- 'docs/**/*.md' > "$DIFF_FILE"
+          # 'docs/**/*.md' to keep markdown-only paths, ':(exclude)docs/.style/**'
+          # so contributor-tooling pages never reach the surgical-reindex payload
+          # on mixed commits. The trigger filter already short-circuits .style-only
+          # pushes; this is defense in depth.
+          git diff --name-status -z "$BEFORE_SHA" "$AFTER_SHA" -- 'docs/**/*.md' ':(exclude)docs/.style/**' > "$DIFF_FILE"
           # Parse the NUL-delimited diff into <path>\t<status> lines.
           # `--name-status -z` uses NUL between fields and between
           # records, with a special twist for renames: the record is

.github/workflows/docs-preview.yaml

@@ -24,6 +24,12 @@ on:
       - reopened
     paths:
       - "docs/**"
+      # docs/.style/** is contributor tooling and never deploys to coder.com.
+      # Skipping the workflow on .style-only PRs avoids posting a preview
+      # link that 404s, since manifest-driven coder.com routing rejects
+      # paths under .style. Mixed PRs still trigger; the selection logic
+      # below filters .style files out of the preview-target pick.
+      - "!docs/.style/**"
 
 concurrency:
   group: docs-preview-${{ github.event.pull_request.number }}
@@ -69,11 +75,18 @@ jobs:
             "repos/${REPO}/pulls/${PR_NUMBER}/files" \
             --jq '.[] | select(.status != "removed") | .filename')
 
-          # Pick the first Markdown file under docs/.  `|| true` keeps
-          # the pipeline from failing when grep finds no matches or
-          # head triggers SIGPIPE under `set -o pipefail`.
+          # Pick the first Markdown file under docs/, excluding the
+          # contributor-tooling subtree at docs/.style/**. Mixed PRs that
+          # touch both .style and a public docs page should land the
+          # preview link on the public page; .style-only PRs already get
+          # short-circuited by the trigger filter above and never reach
+          # this code path.
+          #
+          # `|| true` keeps the pipeline from failing when grep finds no
+          # matches or head triggers SIGPIPE under `set -o pipefail`.
           first_doc=$(printf '%s\n' "$all_files" \
             | grep -E '^docs/.*\.md$' \
+            | grep -v '^docs/\.style/' \
             | head -n 1) || true
 
           if [ -z "$first_doc" ]; then