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/how-to/plan-a-phase.md

@@ -58,6 +58,26 @@ Note: `--research-phase <N>` is a flag on `/gsd-plan-phase`. There is no standal
 
 ---
 
+## Override the planning granularity for one phase
+
+**If you want fewer, larger tasks** for a simple or well-understood phase:
+
+```bash
+/gsd-plan-phase 2 --granularity coarse
+```
+
+**If you want more, smaller tasks** for tighter control over a risky or complex phase:
+
+```bash
+/gsd-plan-phase 2 --granularity fine
+```
+
+`--granularity` accepts `coarse`, `standard`, or `fine`. It overrides all granularity config keys (`granularities.planning`, `granularity`, `planning.granularity`) for this invocation only — no config edit required. Invalid values are rejected immediately with an error.
+
+If you want this granularity applied permanently, set it in config — see [CONFIGURATION.md](../CONFIGURATION.md). For the full flag reference see [COMMANDS.md](../COMMANDS.md).
+
+---
+
 ## Plan vertical feature slices instead of horizontal layers
 
 **If you want tasks organised as thin end-to-end slices** (UI → API → DB per feature) rather than by technical layer: