feat(cli): add /gsd-sync-skills for cross-runtime managed skill sync (#2491)

* fix(tests): update 5 source-text tests to read config-schema.cjs

VALID_CONFIG_KEYS moved from config.cjs to config-schema.cjs in the
drift-prevention companion PR. Tests that read config.cjs source text
and checked for key literal includes() now point to the correct file.

Closes #2480

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

* feat(cli): add /gsd-sync-skills for cross-runtime managed skill sync (#2380)

Adds /gsd-sync-skills command so multi-runtime users can keep gsd-* skill
directories aligned across runtime roots after updating one runtime with gsd-update.

Changes:
- bin/install.js: add --skills-root <runtime> flag that prints the skills root
  path for any supported runtime, reusing the existing getGlobalDir() table.
  Banner is suppressed when --skills-root is used (machine-readable output).
- commands/gsd/sync-skills.md: slash command definition
- get-shit-done/workflows/sync-skills.md: full workflow spec covering argument
  parsing, path resolution via --skills-root, diff computation (CREATE/UPDATE/
  REMOVE/SKIP), dry-run report (default), apply execution, idempotency guarantee,
  and safety rules (only gsd-* touched, dry-run performs no writes).

Safety rules: only gsd-* directories are ever created/updated/removed; non-GSD
skills in destination roots are never touched; --dry-run is the default.

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

---------

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

docs/INVENTORY-MANIFEST.json

@@ -114,6 +114,7 @@
       "/gsd-ui-phase",
       "/gsd-ui-review",
       "/gsd-ultraplan-phase",
+      "/gsd-sync-skills",
       "/gsd-undo",
       "/gsd-update",
       "/gsd-validate-phase",
@@ -192,6 +193,7 @@
       "spike-wrap-up.md",
       "spike.md",
       "stats.md",
+      "sync-skills.md",
       "transition.md",
       "ui-phase.md",
       "ui-review.md",