fix(scripts/check_emdash.sh): skip emdash check when no diff base is available (#26489)

## Problem

`scripts/check_emdash.sh` is a diff gate for pull requests: it resolves
the merge-base against the target branch and only inspects added lines.
When it cannot resolve a base ref, it fell back to scanning **every
tracked file**.

Push builds on release branches hit exactly this case: the `lint` job
checks out with `fetch-depth: 1`, so `origin/main` is absent, and
`GITHUB_BASE_REF` is only set for `pull_request` events. With no base
ref, the whole-tree scan flags the many pre-existing emdash/endash
characters already in the repo and fails `make lint` (`lint/emdash`),
even though the build introduced none of them. Observed on
`release/2.34` CI (run
[27704528068](https://github.com/coder/coder/actions/runs/27704528068/job/81949529546)).

## Fix

When no base ref can be determined (i.e. outside a pull request), skip
the check instead of scanning the entire tree. A full scan remains
available on demand via `scripts/check_emdash.sh --all`.

## Testing

- **No base ref** (release-push simulation, no `GITHUB_BASE_REF`, no
`origin/main`): old script scans all files and fails on a pre-existing
emdash; new script skips and exits 0.
- **PR path** (diff vs merge-base): `OK: no emdash or endash characters
found.`
- **`--all`**: still scans the full tree (flags pre-existing characters
as before).
- `shellcheck` and `shfmt` clean.

## Backports

Backport PRs target `release/2.33` and `release/2.34` (same bug, older
script variant). `release/2.29` and `release/2.32` do **not** contain
`scripts/check_emdash.sh`, so there is nothing to backport there.

<details>
<summary>Decision log</summary>

Considered alternatives to the skip:

1. **Compare against `github.event.before`** on push events. Rejected:
the before-SHA is frequently unreachable in a `fetch-depth: 1` clone,
and wiring it in requires per-workflow env changes that complicate
backports.
2. **Fetch `origin/main` / deepen history** in the release lint job.
Rejected for the same backport-surface reason and because it only masks
the design intent.

The check exists to stop *new* emdashes from landing via PRs; that gate
already ran on the originating PRs. On non-PR builds there is no
meaningful diff base, so skipping is correct and self-contained in the
script (clean to backport). The explicit `--all` mode is preserved for
intentional full-tree audits.

</details>

---
Generated by Coder Agents on behalf of @f0ssel.

scripts/check_emdash.sh

@@ -175,12 +175,19 @@ else
 		exit 1
 	}
 	if [[ -z "$base" ]]; then
-		echo "WARNING: no base ref found, scanning all tracked files." >&2
-		scan_all_files
-	else
-		found=0
-		scan_diff "$base"
+		# No base ref is available outside of pull requests, for
+		# example push builds on release branches with a shallow clone
+		# where neither GITHUB_BASE_REF nor origin/main is present.
+		# The diff check has nothing to compare against, and scanning
+		# every tracked file would flag pre-existing characters that
+		# are unrelated to the change under test. Skip instead; pass
+		# --all to force a full-tree scan.
+		echo "OK: no base ref found (not a pull request); skipping emdash check."
+		echo "    Pass --all to scan every tracked file."
+		exit 0
 	fi
+	found=0
+	scan_diff "$base"
 fi
 
 if [[ "$found" -ne 0 ]]; then