feat(skills): skill lifecycle + ms-* skill set completion (#40, #41, #42)

- scripts/skill.sh: install (bundled or path) / activate / deactivate /
  uninstall (refuses while enabled) / list
- skills-enabled + skills-available dirs under the data root; a skill not
  in skills-enabled is not enabled or available for use
- pi adapter: MOSAIC_SKILLS -> --skill per dir; --no-skills when none
- agent.sh: seat definitions declare skills[]; resolution against
  skills-enabled refuses the launch loudly when missing
- ms-* skills completed (owner-authored canon, hands-off): ms-tools
  adapted to the runtime, ms-file-read/write/agent/conductor bodies
  written in the owner's style; ms-agent-watch + ms-unslop untouched
- tasks/USER.md onboarding fixtures; suite hardening (nested def path,
  user seed, mock-adapter dispatch evidence)

Suites: config 24, task 74, release 14, conductor 17, verify PASS.
RELEASE 0.0.12 packaged; health-gated activation on merge.

Closes #40, closes #41, closes #42

skills/ms-tools/SKILL.md

@@ -29,6 +29,10 @@ recurring failures this skill exists to prevent.
 - node scripts/mosaic-task.mjs retry <runId> re-executes a recorded task as
   a new run. The old record stays.
 
+## Inter-agent communications
+
+- tools/tmux/agent-send.sh    # Send a tmux message to an agent launched in tmux
+
 ## Seats
 
 - scripts/agent.sh <name> launches an interactive seat. It onboards the user