Add action:plan label and transition after plan success

[monkut]

Context

Currently after prepare succeeds, the label action:develop is added (settings.py:18, cli.py:228-229). But the next step is plan, not develop — the label is misleading. Additionally, plan completion doesn't trigger any label or project board transition (cli.py:228-232 only handles prepare and develop).

Current flow:

prepare → +action:develop → plan (no transition) → develop → swap to action:review

Proposal

Add action:plan label and transitions:

New flow:

prepare success  → +action:plan,  project → planning
plan success     → swap action:plan → action:develop, project → ready/todo
develop success  → swap action:develop → action:review, project → in-review

Changes required

1. Add PLAN_LABEL = "action:plan" to settings.py
2. Update transition_issue_to_planning() to add action:plan instead of action:develop
3. Add transition_issue_to_development() function in functions.py that swaps action:plan → action:develop
4. Add plan transition in cli.py after successful plan execution
5. Update label validation to accept action:plan prefix

Rationale

The label state machine should reflect the actual workflow phase. Currently action:develop is added after prepare but plan hasn't run yet — this gives false visibility that the issue is ready for development when it still needs planning.