skill: rename plan to planner

Anthropic now reserves /plan as a UI command, so the Skill tool refuses to
dispatch it. Rename the project's plan skill to planner, update the symlink
under .claude/skills/, and adjust references in CLAUDE.md, DESIGN.md,
skills/README.md, and the cross-references between brainstorm / implement /
audit / fieldtest / fieldtester. Plan files themselves (docs/plans/*.md)
keep their name — only the skill ID changes.

docs/DESIGN.md

@@ -40,7 +40,7 @@ evolving in lockstep with the language:
 - **Examples** (`examples/`): canonical `.ail.json` programs. They are
   specification anchors, not demos — the E2E suite hangs off them.
 - **Skills** (`skills/`): specialised disciplines
-  (`brainstorm`, `plan`, `implement`, `audit`, `debug`,
+  (`brainstorm`, `planner`, `implement`, `audit`, `debug`,
   `fieldtest`) plus the agent rosters they dispatch
   (`skills/<name>/agents/`). They form the project's own
   development methodology and are versioned with the codebase.

docs/specs/2026-05-11-implement-orchestrator-agent.md

@@ -531,7 +531,7 @@ single commit (or one commit per category, atomically).
 - **Worker model downgrade** (cheap models for simple subagents).
   User has explicitly excluded this; all roles stay at Opus 4.7.
 - **Stats aggregation tool** — written once empirical data exists.
-- **Plan-drafter sub-agent** for the `/plan` skill. Separate hebel,
+- **Plan-drafter sub-agent** for the `/planner` skill. Separate hebel,
   separate spec.
 - **`/brainstorm` spec-drafter** — usage statistic shows
   `/brainstorm` at 2%, not cost-effective.