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.md

@@ -54,7 +54,7 @@ Full roster at `agents/gsd-*.md`. The "Primary doc" column flags whether [`docs/
 
 ---
 
-## Commands (83 shipped)
+## Commands (84 shipped)
 
 Full roster at `commands/gsd/*.md`. The groupings below mirror `docs/COMMANDS.md` section order; each row carries the command name, a one-line role derived from the command's frontmatter `description:`, and a link to the source file. `tests/command-count-sync.test.cjs` locks the count against the filesystem.
 
@@ -163,6 +163,7 @@ Full roster at `commands/gsd/*.md`. The groupings below mirror `docs/COMMANDS.md
 | `/gsd-sketch-wrap-up` | Package sketch design findings into a persistent project skill for future build conversations. | [commands/gsd/sketch-wrap-up.md](../commands/gsd/sketch-wrap-up.md) |
 | `/gsd-profile-user` | Generate developer behavioral profile and Claude-discoverable artifacts. | [commands/gsd/profile-user.md](../commands/gsd/profile-user.md) |
 | `/gsd-settings` | Configure GSD workflow toggles and model profile. | [commands/gsd/settings.md](../commands/gsd/settings.md) |
+| `/gsd-settings-advanced` | Power-user configuration — plan bounce, timeouts, branch templates, cross-AI execution, runtime knobs. | [commands/gsd/settings-advanced.md](../commands/gsd/settings-advanced.md) |
 | `/gsd-set-profile` | Switch model profile for GSD agents (quality/balanced/budget/inherit). | [commands/gsd/set-profile.md](../commands/gsd/set-profile.md) |
 | `/gsd-pr-branch` | Create a clean PR branch by filtering out `.planning/` commits. | [commands/gsd/pr-branch.md](../commands/gsd/pr-branch.md) |
 | `/gsd-sync-skills` | Sync managed GSD skill directories across runtime roots for multi-runtime users. | [commands/gsd/sync-skills.md](../commands/gsd/sync-skills.md) |
@@ -173,7 +174,7 @@ Full roster at `commands/gsd/*.md`. The groupings below mirror `docs/COMMANDS.md
 
 ---
 
-## Workflows (81 shipped)
+## Workflows (82 shipped)
 
 Full roster at `get-shit-done/workflows/*.md`. Workflows are thin orchestrators that commands reference internally; most are not read directly by end users. Rows below map each workflow file to its role (derived from the `<purpose>` block) and, where applicable, to the command that invokes it.
 
@@ -243,6 +244,7 @@ Full roster at `get-shit-done/workflows/*.md`. Workflows are thin orchestrators
 | `secure-phase.md` | Retroactive threat-mitigation audit for a completed phase. | `/gsd-secure-phase` |
 | `session-report.md` | Session report — token usage, work summary, outcomes. | `/gsd-session-report` |
 | `settings.md` | Configure GSD workflow toggles and model profile. | `/gsd-settings`, `/gsd-set-profile` |
+| `settings-advanced.md` | Configure GSD power-user knobs — plan bounce, timeouts, branch templates, cross-AI execution, runtime knobs. | `/gsd-settings-advanced` |
 | `ship.md` | Create PR, run review, and prepare for merge after verification. | `/gsd-ship` |
 | `sketch.md` | Explore design directions through throwaway HTML mockups with 2-3 variants per sketch. | `/gsd-sketch` |
 | `sketch-wrap-up.md` | Curate sketch findings and package them as a persistent `sketch-findings-[project]` skill. | `/gsd-sketch-wrap-up` |