feat(#2081): CI lint for comment blocks over 30 lines

Flags any contiguous comment block longer than 30 lines — the threshold
above which a why/impl-notes block should move to docs/ rather than live
in-code (the #2077 rubric, made self-enforcing).

- scripts/check-comment-blocks.sh: git+awk, mirrors check-issue-refs.sh's
  capture-output shape (robust to xargs batching). Handles # line comments
  (nix/sh), // line comments (rs/js/ts), and /* */ (rs/js/ts/css) + <!-- -->
  (html) block comments. Blank separates line-comment blocks; a blank inside
  a /* */ / <!-- --> block stays part of it. lint:allow-long-comment escape
  hatch. Threshold is a tunable constant.
- ci.yml: own 'comment-block lint' job, kept OUT of required checks while the
  tree settles (red signal, not a merge gate), like the tracker-tag lint.

Current tree has 6 blocks > 30 (all in the frontend + hive-c0re-core #2077
slices, none in nix/infra): matrix-accounts.js, permissions.js,
stream-worker.js, terminal.js, hivectl.rs:1036, assets.rs. Non-required, so
non-blocking — they're the remaining #2077 targets for those area owners.

.forgejo/workflows/ci.yml

@@ -40,3 +40,18 @@ jobs:
         # merge. Promote to a required check once the tree is clean.
         # See scripts/check-issue-refs.sh.
         run: sh scripts/check-issue-refs.sh
+
+  comment-blocks:
+    name: comment-block lint
+    runs-on: [hive-ci]
+    # Pure git+awk — seconds.
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v3
+      - name: lint
+        # Flags contiguous comment blocks over 30 lines (a giant prose
+        # block belongs in docs/ as implementation notes, not in source).
+        # Own job, kept out of required checks while the tree settles: a
+        # hit reds this check without blocking merge. Promote to required
+        # once clean. See scripts/check-comment-blocks.sh.
+        run: sh scripts/check-comment-blocks.sh