fix(#1145): implement query user-story.validate handler (#1193)

* fix(#1145): implement query user-story.validate handler

`query user-story.validate` was a phantom command invoked by mvp-phase.md
(line 102) and verify-work.md (line 170) but had no CJS handler. Every
call exited with "Unknown command: user-story". The dotted-form dispatcher
strips the `query` prefix, splits on `.`, yielding command='user-story'
which fell through to the `default:` case with no registered capability
handler.

Adds a `case 'user-story':` handler inline in gsd-tools.cjs (same pattern
as the #1140 fix in PR #1148). The handler:

- Validates "As a [role], I want to [capability], so that [outcome]."
- Uses \S anchors to require non-whitespace content in each slot
  (whitespace-only slots like "As a  , I want to ..." now correctly
  return valid:false — found by adversarial Codex review)
- Returns { valid: boolean, errors: string[], slots: {role, capability,
  outcome} | null }
- Supports --pick valid for bare boolean output (verify-work.md usage)
- Added 'user-story' to SKIP_ROOT_RESOLUTION (pure string validation)
- Added 'user-story' to TOP_LEVEL_USAGE command list

Regression tests added to tests/commands.test.cjs (per lint-regression-
test-names policy; new bug-NNNN standalone files are banned).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(changeset): backfill PR number for #1145 fix

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/brave-lions-revolt.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 1193
+---
+**`query user-story.validate` now works** — `mvp-phase` and `verify-work` workflows both invoked this command to validate "As a / I want to / so that" user stories, but no CJS handler existed; every call errored with "Unknown command: user-story". (#1193)