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.

skills/implement/SKILL.md

@@ -90,7 +90,7 @@ reviewer; loop until `compliant`.
 
 If `unclear`: the task text is ambiguous. Resolve at the orchestrator
 level — either by re-stating the task to the implementer with the
-ambiguity removed, or by bouncing back to `plan` if the spec itself
+ambiguity removed, or by bouncing back to `planner` if the spec itself
 is the source of the ambiguity.
 
 #### 2.4 — Code quality review
@@ -152,7 +152,7 @@ summary:
 
 | Source | Carrier |
 |--------|---------|
-| from `plan` | path to `docs/plans/<iteration>.md` + optional task focus |
+| from `planner` | path to `docs/plans/<iteration>.md` + optional task focus |
 | from `debug` | RED-test path + cause summary + minimal-fix constraint |
 
 `implement` produces: per-task commits + JOURNAL entry. No further
@@ -202,7 +202,7 @@ Reviewer subagents typically need at least the standard model.
     is the diff well-built?
   - `skills/implement/agents/ailang-tester.md` — Step 3, E2E coverage
 - **Input sources:**
-  - `skills/plan/SKILL.md` — produces the plan files this skill
+  - `skills/planner/SKILL.md` — produces the plan files this skill
     consumes
   - `skills/debug/SKILL.md` — produces the RED-test handoff for
     mini-mode