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/explanation/multi-agent-orchestration.md

@@ -222,7 +222,7 @@ input rather than conversation history.
 ## Related
 
 - [Context engineering](context-engineering.md) — the upstream principle that
-  motivates this design
+  motivates this design; see also [Lifecycle hooks and context headroom](context-engineering.md#lifecycle-hooks-and-context-headroom) for how per-turn headroom tracking and forked-context skills extend the same principle at runtime
 - [Configure model profiles](../how-to/configure-model-profiles.md) — how to
   assign model tiers per agent
 - [Configuration reference](../CONFIGURATION.md) — full `config.json` schema