design: ~/.apc/skills/ as single source of truth — dir-level symlinks, no inline skills

[FZ2000]

Design Decision

Current behaviour (broken)

• apc collect stores skills inline in ~/.apc/cache/bundle.json
• apc install saves to ~/.apc/skills/ then copies (fixed in fix(#33): apc install symlinks skills instead of copying them #58) or per-skill symlinks into each tool
• apc sync creates per-skill symlinks from <tool_skills_dir>/<name> → ~/.apc/skills/<name>
• New apc install after sync → does not auto-propagate to synced tools

Desired behaviour

1. ~/.apc/skills/ is the single source of truth for all skills — installed and collected
2. apc collect writes extracted skills to ~/.apc/skills/<name>/SKILL.md (not inline cache)
3. apc install only saves to ~/.apc/skills/ — nothing else
4. apc sync establishes a dir-level symlink once: <tool_skills_dir> → ~/.apc/skills/
5. After sync, any apc install automatically appears in the tool — no re-sync needed

Per-tool plan

Tool	Skills dir	Approach
OpenClaw	~/.openclaw/skills/	✅ Dir-level symlink (exclusive dir)
Claude Code	~/.claude/skills/	✅ Dir-level symlink (exclusive dir)
Cursor	~/.cursor/rules/	⚠️ Per-skill symlinks (mixed dir — user may have other rules)
Windsurf	None	N/A
Gemini	None	N/A
Copilot	~/.github/instructions/	⚠️ Per-skill symlinks (project-level, mixed)

Code changes

• src/appliers/base.py: add SKILL_DIR_EXCLUSIVE flag + sync_skills_dir() method
• src/appliers/openclaw.py + claude.py: set SKILL_DIR_EXCLUSIVE = True
• src/collect.py: write skills to ~/.apc/skills/ via save_skill_file() instead of inline cache
• src/sync_helpers.py: use sync_skills_dir() for exclusive tools; keep link_skills() for others
• src/install.py: remove _link_skill_to_targets() — save to ~/.apc/skills/ only; exclusive tools get it automatically, per-skill tools document that re-sync is needed
• src/cache.py: deprecate save_skills()/load_skills() for skills (keep for MCP/memory)