feat: add skills system for extensible capability packages

Implement a three-tier skill system (bundled/managed/workspace) that
extends Flynn's abilities via SKILL.md instructions injected into the
system prompt.

- SkillManifest/Skill types with requirements gating (OS, binaries, env)
- Loader: discovers skills from directories, validates manifests,
  checks system requirements, infers manifest from SKILL.md if missing
- SkillRegistry: holds skills, generates system prompt additions,
  supports override by name (workspace > managed > bundled)
- SkillInstaller: copies/removes skills in managed directory with
  upgrade support
- Config: add skills.workspace_dir, managed_dir, bundled_dir options
- Daemon: loads all skills at startup, injects available skill
  instructions into the system prompt
- Tests: 45 new tests (loader 22, registry 11, installer 12)

src/skills/index.ts

@@ -0,0 +1,4 @@
+export type { SkillTier, SkillRequirements, SkillManifest, Skill } from './types.js';
+export { checkRequirements, loadSkill, discoverSkills, loadAllSkills } from './loader.js';
+export { SkillRegistry } from './registry.js';
+export { SkillInstaller } from './installer.js';