## What
`clidocgen` and the new `configdocgen` (coder/coder#26824) both carried
a byte-identical `prepareEnv()` that unsets `CODER_*` and pins
`CLIDOCGEN_*` / `TMPDIR` so generated docs don't embed the generating
host's home directory.
This extracts it to `scripts/docgenenv.Prepare()` and migrates
`clidocgen`.
## Why
Duplication flagged during review of #26824. `configdocgen` adopts the
shared helper in that PR, removing its copy.
## Risk
Behavior-preserving: regenerating the CLI reference (`make
docs/reference/cli/index.md`) yields no diff, and `make pre-commit`
passes (`lint/go`, `lint/ts`, `build`). A focused unit test pins the
`Prepare()` contract, and `_test.go` files are excluded from
`CLIDOCGEN_INPUTS` so test edits don't mark the generated docs stale.
<details>
<summary>CI status — blocked by an unrelated <code>main</code> breakage
(#24993)</summary>
All red checks on this PR are inherited from `main`, not caused by these
changes. This PR touches only `Makefile` and
`scripts/{clidocgen,docgenenv}`; it does not touch Helm.
`main` went red at `d0f68cb9b0` ("feat: add listenerset", #24993, merged
~18:26 UTC). The committed
`helm/coder/tests/testdata/listenerset*.golden` files don't match what
`helm template` renders, so:
- **`gen`** regenerates those goldens, and the unstaged-files check
fails.
- **`test-go-pg` (ubuntu-latest, pg-17) and `test-go-race-pg`** fail
only on `TestRenderChart/{coder,default}/listenerset[_redirect]` (golden
mismatch; the test prints "Run with -update to update golden files").
The same `test-go-pg` job passes on macOS and Windows, where the Helm
render test is skipped, and `scripts/docgenenv` reports `ok` on the
failing runners.
Base commit `14a61041d9` was green; `main` is red from `d0f68cb9b0`
onward. These checks clear once `main` is fixed and this branch is
updated. `fmt`, `lint`, `Storybook`, `check-build`, and `test-e2e` are
green.
</details>
---
🤖 Opened by Coder Agents on behalf of @nickvigilante.
---------
Co-authored-by: Cian Johnston <cian@coder.com>
