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/CONFIGURATION.md

@@ -114,6 +114,7 @@ GSD stores project settings in `.planning/config.json`. Created during `/gsd-new
 | `model_profile` | enum | `quality`, `balanced`, `budget`, `inherit` | `balanced` | Model tier for each agent (see [Model Profiles](#model-profiles)) |
 | `project_code` | string | any short string | (none) | Prefix for phase directory names (e.g., `"ABC"` produces `ABC-01-setup/`). Added in v1.31 |
 | `response_language` | string | language code | (none) | Language for agent responses (e.g., `"pt"`, `"ko"`, `"ja"`). Propagates to all spawned agents for cross-phase language consistency. Added in v1.32 |
+| `context_window` | number | any integer | `200000` | Context window size in tokens. Set `1000000` for 1M-context models (e.g., `claude-opus-4-7[1m]`). Values `>= 500000` enable adaptive context enrichment (full-body reads of prior SUMMARY.md, deeper anti-pattern reads). Configured via `/gsd-settings-advanced`. |
 | `context_profile` | string | `dev`, `research`, `review` | (none) | Execution context preset that applies a pre-configured bundle of mode, model, and workflow settings for the current type of work. Added in v1.34 |
 | `claude_md_path` | string | any file path | `./CLAUDE.md` | Custom output path for the generated CLAUDE.md file. Useful for monorepos or projects that need CLAUDE.md in a non-root location. Defaults to `./CLAUDE.md` at the project root. Added in v1.36 |
 | `claude_md_assembly.mode` | enum | `embed`, `link` | `embed` | Controls how managed sections are written into CLAUDE.md. `embed` (default) inlines content between GSD markers. `link` writes `@.planning/<source-path>` instead — Claude Code expands the reference at runtime, reducing CLAUDE.md size by ~65% on typical projects. `link` only applies to sections that have a real source file; `workflow` and fallback sections always embed. Per-block overrides: `claude_md_assembly.blocks.<section>` (e.g. `claude_md_assembly.blocks.architecture: link`). Added in v1.38 |