feat: platform-conditional skill loading + Apple/macOS skills

Add a 'platforms' field to SKILL.md frontmatter that restricts skills
to specific operating systems. Skills with platforms: [macos] only
appear in the system prompt, skills_list(), and slash commands on macOS.
Skills without the field load everywhere (backward compatible).

Implementation:
- skill_matches_platform() in tools/skills_tool.py — core filter
- Wired into all 3 discovery paths: prompt_builder.py, skills_tool.py,
  skill_commands.py
- 28 new tests across 3 test files

New bundled Apple/macOS skills (all platforms: [macos]):
- imessage — Send/receive iMessages via imsg CLI
- apple-reminders — Manage Reminders via remindctl CLI
- apple-notes — Manage Notes via memo CLI
- findmy — Track devices/AirTags via AppleScript + screen capture

Docs updated: CONTRIBUTING.md, AGENTS.md, creating-skills.md,
skills.md (user guide)

agent/skill_commands.py

@@ -22,7 +22,7 @@ def scan_skill_commands() -> Dict[str, Dict[str, Any]]:
     global _skill_commands
     _skill_commands = {}
     try:
-        from tools.skills_tool import SKILLS_DIR, _parse_frontmatter
+        from tools.skills_tool import SKILLS_DIR, _parse_frontmatter, skill_matches_platform
         if not SKILLS_DIR.exists():
             return _skill_commands
         for skill_md in SKILLS_DIR.rglob("SKILL.md"):
@@ -31,6 +31,9 @@ def scan_skill_commands() -> Dict[str, Dict[str, Any]]:
             try:
                 content = skill_md.read_text(encoding='utf-8')
                 frontmatter, body = _parse_frontmatter(content)
+                # Skip skills incompatible with the current OS platform
+                if not skill_matches_platform(frontmatter):
+                    continue
                 name = frontmatter.get('name', skill_md.parent.name)
                 description = frontmatter.get('description', '')
                 if not description: