feat: /gsd-settings-integrations — configure third-party search and review integrations (closes #2529) (#2604)

* feat(#2529): /gsd-settings-integrations — third-party integrations command

Adds /gsd-settings-integrations for configuring API keys, code-review CLI
routing, and agent-skill injection. Distinct from /gsd-settings (workflow
toggles) because these are connectivity, not pipeline shape.

Three sections:
- Search Integrations: brave_search / firecrawl / exa_search API keys,
  plus search_gitignored toggle.
- Code Review CLI Routing: review.models.{claude,codex,gemini,opencode}
  shell-command strings.
- Agent Skills Injection: agent_skills.<agent-type> free-text input,
  validated against [a-zA-Z0-9_-]+.

Security:
- New secrets.cjs module with ****<last-4> masking convention.
- cmdConfigSet now masks value/previousValue in CLI output for secret keys.
- Plaintext is written only to .planning/config.json; never echoed to
  stdout/stderr, never written to audit/log files by this flow.
- Slug validators reject path separators, whitespace, shell metacharacters.

Tests (tests/settings-integrations.test.cjs — 25 cases):
- Artifact presence / frontmatter.
- Field round-trips via gsd-tools config-set for all four search keys,
  review.models.<cli>, agent_skills.<agent-type>.
- Config-merge safety: unrelated keys preserved across writes.
- Masking: config-set output never contains plaintext sentinel.
- Logging containment: plaintext secret sentinel appears only in
  config.json under .planning/, nowhere else on disk.
- Negative: path-traversal, shell-metachar, and empty-slug rejected.
- /gsd:settings workflow mentions /gsd:settings-integrations.

Docs:
- docs/COMMANDS.md: new command entry with security note.
- docs/CONFIGURATION.md: integration settings section (keys, routing,
  skills injection) with masking documentation.
- docs/CLI-TOOLS.md: reviewer CLI routing and secret-handling sections.
- docs/INVENTORY.md + INVENTORY-MANIFEST.json regenerated.

Closes #2529

* fix(#2529): mask secrets in config-get; address CodeRabbit review

cmdConfigGet was emitting plaintext for brave_search/firecrawl/exa_search.
Apply the same isSecretKey/maskSecret treatment used by config-set so the
CLI surface never echoes raw API keys; plaintext still lives only in
config.json on disk.

Also addresses CodeRabbit review items in the same PR area:
- #3127146188: config-get plaintext leak (root fix above)
- #3127146211: rename test sentinels to concat-built markers so secret
  scanners stop flagging the test file. Behavior preserved.
- #3127146207: add explicit 'text' language to fenced code blocks (MD040).
- nitpick: unify masked-value wording in read_current legend
  ('****<last-4>' instead of '**** already set').
- nitpick: extend round-trip test to cover search_gitignored toggle.

New regression test 'config-get masks secrets and never echoes plaintext'
verifies the fix for all three secret keys.

* docs(#2529): bump INVENTORY counts post-rebase (commands 84→85, workflows 82→83)

* fix(test): bump CLI Modules count 27→28 after rebase onto main (CI #24811455435)

PR #2604 was rebased onto main before #2605 (drift.cjs) merged. The
pull_request CI runs against the merge ref (refs/pull/2604/merge),
which now contains 28 .cjs files in get-shit-done/bin/lib/, but
docs/INVENTORY.md headline still said "(27 shipped)".

inventory-counts.test.cjs failed with:
  AssertionError: docs/INVENTORY.md "CLI Modules (27 shipped)" disagrees
  with get-shit-done/bin/lib/ file count (28)

Rebased branch onto current origin/main (picks up drift.cjs row, which
was already added by #2605) and bumped the headline to 28.

Full suite: 5200/5200 pass.

docs/COMMANDS.md

@@ -1075,6 +1075,35 @@ Current values are pre-selected; an empty input keeps the existing value. Numeri
 
 See [CONFIGURATION.md](CONFIGURATION.md) for the full schema and defaults.
 
+### `/gsd-settings-integrations`
+
+Interactive configuration of third-party integrations and cross-tool routing.
+Distinct from `/gsd-settings` (workflow toggles) — this command handles
+connectivity: API keys, reviewer CLI routing, and agent-skill injection.
+
+Covers:
+
+- **Search integrations:** `brave_search`, `firecrawl`, `exa_search` API keys,
+  and the `search_gitignored` toggle.
+- **Code-review CLI routing:** `review.models.{claude,codex,gemini,opencode}`
+  — a shell command per reviewer flavor.
+- **Agent-skill injection:** `agent_skills.<agent-type>` — skill names
+  injected into an agent's spawn frontmatter. Agent-type slugs are validated
+  against `[a-zA-Z0-9_-]+` so path separators and shell metacharacters are
+  rejected.
+
+API keys are stored plaintext in `.planning/config.json` but displayed masked
+(`****<last-4>`) in every interactive output, confirmation table, and
+`config-set` stdout/stderr line. Plaintext is never echoed, never logged,
+and never written to any file outside `config.json` by this workflow.
+
+```bash
+/gsd-settings-integrations           # Interactive config (three sections)
+```
+
+See [`docs/CONFIGURATION.md`](CONFIGURATION.md) for the per-field reference and
+[`docs/CLI-TOOLS.md`](CLI-TOOLS.md) for the reviewer-CLI routing contract.
+
 ### `/gsd-set-profile`
 
 Quick profile switch.