docs(#849): capture 1.4.0 release features across the docs base (#850)

Diataxis review of the 1.4.0 content (52 changesets, multi-runtime maturation
plus native packaging and new flags) against the existing docs base found most
per-feature docs already landed with their PRs. Fill the four remaining gaps,
each in its Diataxis quadrant:

- Reference: FEATURES.md Feature #36 (Multi-Runtime Support) updated in place
  with 1.4.0 additions — native skills emission (Cline/Kilo/OpenCode), new
  slash-command surfaces (CodeBuddy/Augment/Cursor), cross-runtime lifecycle
  hooks for context-headroom tracking, and the Gemini CLI extension package.
- Reference: CONFIGURATION.md gains a dedicated worktree.baseRef entry (values,
  .claude/settings.local.json location, auto-set-on-install behaviour).
- How-to: plan-a-phase.md gains an 'override planning granularity for one phase'
  section for the --granularity flag.
- Explanation: context-engineering.md gains a 'Lifecycle hooks and context
  headroom' section (the why of lifecycle hooks + forked context), cross-linked
  from multi-agent-orchestration.md.

Docs-only; documents already-shipped features, so no changeset required
(docs/ is not in the changeset-lint user-facing prefixes).

Closes #849

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/CONFIGURATION.md

@@ -248,7 +248,7 @@ All workflow toggles follow the **absent = enabled** pattern. If a key is missin
 | `workflow.max_discuss_passes` | number | `3` | Maximum number of question rounds in discuss-phase before the workflow stops asking. Useful in headless/auto mode to prevent infinite discussion loops. |
 | `workflow.skip_discuss` | boolean | `false` | When `true`, `/gsd-autonomous` bypasses the discuss-phase entirely, writing minimal CONTEXT.md from the ROADMAP phase goal. Useful for projects where developer preferences are fully captured in PROJECT.md/REQUIREMENTS.md. Added in v1.28 |
 | `workflow.text_mode` | boolean | `false` | Replaces AskUserQuestion TUI menus with plain-text numbered lists. Required for Claude Code remote sessions (`/rc` mode) where TUI menus don't render. Can also be set per-session with `--text` flag on discuss-phase. Added in v1.28 |
-| `workflow.use_worktrees` | boolean | `true` | When `false`, disables git worktree isolation for parallel execution. Users who prefer sequential execution or whose environment does not support worktrees can disable this. Added in v1.31. **Branch-divergence note:** when your branch is ahead of `origin/HEAD`, GSD auto-degrades to sequential and prints a warning. Set `worktree.baseRef:"head"` in `.claude/settings.local.json` (run `node gsd-tools.cjs worktree set-baseref`) to restore parallel execution. See [Fix the worktree base-mismatch (exit 42) error](how-to/fix-worktree-base-mismatch.md). |
+| `workflow.use_worktrees` | boolean | `true` | When `false`, disables git worktree isolation for parallel execution. Users who prefer sequential execution or whose environment does not support worktrees can disable this. Added in v1.31. **Branch-divergence note:** when your branch has diverged from `origin/HEAD`, GSD auto-degrades to sequential and prints a warning. See [`worktree.baseRef`](#worktree-settings) to restore parallel execution on a diverged branch. |
 | `workflow.worktree_skip_hooks` | boolean | `false` | When `true`, executor agents in worktree mode pass `--no-verify` (skipping pre-commit hooks) and post-wave hook validation runs against the merged result instead. Opt-in escape hatch for projects whose hooks cannot run in agent worktrees. Default `false` runs hooks on every commit (#2924). |
 | `workflow.code_review` | boolean | `true` | Enable `/gsd-code-review` and `/gsd-code-review --fix` commands. When `false`, the commands exit with a configuration gate message. Added in v1.34 |
 | `workflow.code_review_depth` | string | `standard` | Default review depth for `/gsd-code-review`: `quick` (pattern-matching only), `standard` (per-file analysis), or `deep` (cross-file with import graphs). Can be overridden per-run with `--depth=`. Added in v1.34 |
@@ -276,6 +276,14 @@ All workflow toggles follow the **absent = enabled** pattern. If a key is missin
 | `workflow.build_command` | string | (none) | Shell command to build the project in the post-merge build gate (Step A of step 5.6 in execute-phase). When unset, the gate auto-detects: Xcode (`.xcodeproj` present) → `xcodebuild build`, `Makefile` with `build:` target → `make build`, Justfile → `just build`, `Cargo.toml` → `cargo build`, `go.mod` → `go build ./...`, Python → `python -m py_compile`, `package.json` with `build` script → `npm run build`. Runs with a 5-minute timeout; failure increments `WAVE_FAILURE_COUNT`. Added in v1.39 |
 | `workflow.test_command` | string | (none) | Shell command to run the project's test suite in the post-merge test gate (Step B of step 5.6 in execute-phase) and the regression gate. When unset, the gate auto-detects: Xcode (`.xcodeproj` present) → `xcodebuild test`, `Makefile` with `test:` target → `make test`, Justfile → `just test`, `package.json` → `npm test`, `Cargo.toml` → `cargo test`, `go.mod` → `go test ./...`, Python → `python -m pytest`. Runs with a 5-minute timeout; failure increments `WAVE_FAILURE_COUNT`. Added in v1.39 |
 
+## Worktree Settings
+
+> **File:** `.claude/settings.local.json` — not `.planning/config.json`. Unlike all other keys in this reference, `worktree.*` settings live in the Claude Code runtime settings file. Fresh installs and upgrades auto-set `worktree.baseRef: "head"` there (no-clobber) when `workflow.use_worktrees` is enabled. The key can also be set via `gsd-tools worktree set-baseref`.
+
+| Setting | Type | Default | Description |
+|---------|------|---------|-------------|
+| `worktree.baseRef` | string | (unset) | Controls which ref the worktree-based parallel executor uses as the base when creating new phase/wave worktrees. When unset, the executor bases new worktrees on the repository default branch (`origin/HEAD`); if the current branch has diverged, execute-phase auto-degrades to sequential execution rather than halting (as of v1.4.0). Set to `"head"` to base new worktrees on the local `HEAD` instead — the appropriate choice when working on a branch that has diverged from the default branch, as it prevents the exit-42 base-mismatch halt and allows wave-based parallel execution to proceed normally. See [Fix the worktree base-mismatch (exit 42) error](how-to/fix-worktree-base-mismatch.md). |
+
 ## Code Quality Settings
 
 The `code_quality.*` namespace gates optional structural-analysis tooling that augments `/gsd-code-review`. Settings are additive: each tool is independently opt-in and off by default.