Rename plugin to plankit; noun-first command naming

Drops the dash from the plugin name (plan-kit → plankit) and reorganizes
all commands and skills to start with the noun of the PlanKit level they
operate on:

  /plankit:idea-dump          (was /plan-kit:capture-idea)
  /plankit:roadmap-planning   (was /plan-kit:plan-roadmap)
  /plankit:mission-breakdown  (was /plan-kit:define-missions)
  /plankit:init               (was /plan-kit:init)

Skill folders mirror the command names: idea-dump, roadmap-planning,
mission-breakdown. Dashboard and done skills keep their names. Content
folder conventions (PlanKit/, plan-kit/) stay unchanged — only plugin-side
naming moves.

Bumps plugin version to 0.4.0.

Rename Steps → Missions; mention TandemKit as mission runner

Adopts the same terminology used by TandemKit: the phase previously
called "Steps" is now "Missions" throughout. Rename detail-steps →
detail-missions, define-steps → define-missions, Steps: → Missions: in
Progress.md, step files → mission files in Features/, and all
accompanying documentation and examples.

Also adds a small section in README and in the detail-missions skill
pointing users at TandemKit for autonomously executing a mission
through a Planner / Generator / Evaluator loop with Claude + Codex.

Bumps plugin version to 0.3.0.