feat: /gsd-settings-advanced — power-user config tuning command (closes #2528) (#2603)

* feat: /gsd-settings-advanced — power-user config tuning command (closes #2528)

Adds a second-tier interactive configuration command covering the power-user
knobs that don't belong in the common-case /gsd-settings prompt. Six sectioned
AskUserQuestion batches cover planning, execution, discussion, cross-AI, git,
and runtime settings (19 config keys total). Current values are pre-selected;
numeric fields reject non-numeric input; writes route through
gsd-sdk query config-set so unrelated keys are preserved.

- commands/gsd/settings-advanced.md — command entry
- get-shit-done/workflows/settings-advanced.md — six-section workflow
- get-shit-done/workflows/settings.md — advertise advanced command
- get-shit-done/bin/lib/config-schema.cjs — add context_window to VALID_CONFIG_KEYS
- docs/COMMANDS.md, docs/CONFIGURATION.md, docs/INVENTORY.md — docs + inventory
- tests/gsd-settings-advanced.test.cjs — 81 tests (files, frontmatter,
  field coverage, pre-selection, merge-preserves-siblings, VALID_CONFIG_KEYS
  membership, confirmation table, /gsd-settings cross-link, negative scenarios)

All 5073 tests pass; coverage 88.66% (>= 70% threshold).

* docs(settings-advanced): clarify per-field numeric bounds and label fenced blocks

Addresses CodeRabbit review on PR #2603:
- Numeric-input rule now states min is field-specific: plan_bounce_passes
  and max_discuss_passes require >= 1; other numeric fields accept >= 0.
  Resolves the inconsistency between the global rule and the field-level
  prompts (CodeRabbit comment 3127136557).
- Adds 'text' fence language to seven previously unlabeled code blocks in
  the workflow (six AskUserQuestion sections plus the confirmation banner)
  to satisfy markdownlint MD040 (CodeRabbit comment 3127136561).

* test(settings-advanced): tighten section assertion, fix misleading test name, add executable numeric-input coverage

Addresses CodeRabbit review on PR #2603:
- Required section list now asserts the full 'Runtime / Output' heading
  rather than the looser 'Runtime' substring (comment 3127136564).
- Renames the subagent_timeout coercion test to match the actual key
  under test (was titled 'context_window' but exercised
  workflow.subagent_timeout — comment 3127136573).
- Adds two executable behavioral tests at the config-set boundary
  (comment 3127136579):
  * Non-numeric input on a numeric key currently lands as a string —
    locks in that the workflow's AskUserQuestion re-prompt loop is the
    layer responsible for type rejection. If a future change adds CLI-side
    numeric validation, the assertion flips and the test surfaces it.
  * Numeric string on workflow.max_discuss_passes is coerced to Number —
    locks in the parser invariant for a second numeric key.

docs/INVENTORY-MANIFEST.json

@@ -1,5 +1,5 @@
 {
-  "generated": "2026-04-20",
+  "generated": "2026-04-22",
   "families": {
     "agents": [
       "gsd-advisor-researcher",
@@ -103,6 +103,7 @@
       "/gsd-session-report",
       "/gsd-set-profile",
       "/gsd-settings",
+      "/gsd-settings-advanced",
       "/gsd-ship",
       "/gsd-sketch",
       "/gsd-sketch-wrap-up",
@@ -110,11 +111,11 @@
       "/gsd-spike",
       "/gsd-spike-wrap-up",
       "/gsd-stats",
+      "/gsd-sync-skills",
       "/gsd-thread",
       "/gsd-ui-phase",
       "/gsd-ui-review",
       "/gsd-ultraplan-phase",
-      "/gsd-sync-skills",
       "/gsd-undo",
       "/gsd-update",
       "/gsd-validate-phase",
@@ -185,6 +186,7 @@
       "scan.md",
       "secure-phase.md",
       "session-report.md",
+      "settings-advanced.md",
       "settings.md",
       "ship.md",
       "sketch-wrap-up.md",