docs: replace retired /gsd-intel with /gsd-map-codebase --query (#3258) (#3260)

* test: forbid stale /gsd-intel references in workflow/reference docs (#3258)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: replace retired /gsd-intel with /gsd-map-codebase --query (#3258)

Fixes 5 stale references across the two primary source files called out in
the issue. PR #2790 folded /gsd-intel into /gsd-map-codebase --query; these
prose surfaces were not updated at that time.

Fixes #3258

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: fix additional stale /gsd-intel references found in adversarial sweep (#3258)

Sweep found 7 more occurrences in docs/INVENTORY.md (x2), docs/USER-GUIDE.md (x4),
docs/FEATURES.md (x2), and agents/gsd-intel-updater.md (x2). All replaced with
/gsd-map-codebase --query. The gsd-intel-updater agent name itself (without leading
slash) is intentionally preserved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* changeset: pr=3260 for #3258

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: fail loudly on unreadable files in bug-3258 regression scan (CR finding)

Replace silent early-return on readFileSync failure with an explicit
throw so unreadable files surface as test failures rather than skipped
coverage gaps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.changeset/bold-elks-zip.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3260
+---
+**`/gsd-settings` Intel question now points to the correct command** — was telling users to use the retired `/gsd-intel` (folded into `/gsd-map-codebase --query` by #2790). Same correction applied to `references/planning-config.md`, `docs/USER-GUIDE.md`, `docs/FEATURES.md`, `docs/INVENTORY.md`, and `agents/gsd-intel-updater.md`. No backend change.