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)

website/docs/user-guide/features/skills.md

@@ -50,6 +50,7 @@ The agent only loads the full skill content when it actually needs it.
 name: my-skill
 description: Brief description of what this skill does
 version: 1.0.0
+platforms: [macos, linux]     # Optional — restrict to specific OS platforms
 metadata:
   hermes:
     tags: [python, automation]
@@ -72,6 +73,23 @@ Trigger conditions for this skill.
 How to confirm it worked.
 ```
 
+### Platform-Specific Skills
+
+Skills can restrict themselves to specific operating systems using the `platforms` field:
+
+| Value | Matches |
+|-------|---------|
+| `macos` | macOS (Darwin) |
+| `linux` | Linux |
+| `windows` | Windows |
+
+```yaml
+platforms: [macos]            # macOS only (e.g., iMessage, Apple Reminders, FindMy)
+platforms: [macos, linux]     # macOS and Linux
+```
+
+When set, the skill is automatically hidden from the system prompt, `skills_list()`, and slash commands on incompatible platforms. If omitted, the skill loads on all platforms.
+
 ## Skill Directory Structure
 
 ```