Completes the decision-presentation story: the CLI that deploys the plugin's
present-decision skill onto the 3 agents on demand (the owner's "plugin installs
skills via the CLI on claude/codex/gemini-agy"). Modeled on h2a's installer.

- track install-skills --host claude|codex|gemini|agy|all [--scope user|project]
  [--force]. claude -> ~/.claude/skills/; codex -> ~/.codex/skills/ (+ bounded
  AGENTS.md pointer on --scope project); gemini/agy -> ~/.gemini/commands/
  present-decision.toml (translated). Single source = the in-repo skills/; graceful
  + idempotent (a differing file is reported + skipped unless --force); never
  creates an absent repo entrypoint. HOME injectable for tests; no real dir touched.

Additive; event contract / write path / P0 guard untouched. 428 tests. 0.10.6.