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)

CONTRIBUTING.md

@@ -325,6 +325,9 @@ description: Brief description (shown in skill search results)
 version: 1.0.0
 author: Your Name
 license: MIT
+platforms: [macos, linux]          # Optional — restrict to specific OS platforms
+                                   #   Valid: macos, linux, windows
+                                   #   Omit to load on all platforms (default)
 metadata:
   hermes:
     tags: [Category, Subcategory, Keywords]
@@ -351,6 +354,18 @@ Known failure modes and how to handle them.
 How the agent confirms it worked.
 ```
 
+### Platform-specific skills
+
+Skills can declare which OS platforms they support via the `platforms` frontmatter field. Skills with this field are automatically hidden from the system prompt, `skills_list()`, and slash commands on incompatible platforms.
+
+```yaml
+platforms: [macos]            # macOS only (e.g., iMessage, Apple Reminders)
+platforms: [macos, linux]     # macOS and Linux
+platforms: [windows]          # Windows only
+```
+
+If the field is omitted or empty, the skill loads on all platforms (backward compatible). See `skills/apple/` for examples of macOS-only skills.
+
 ### Skill guidelines
 
 - **No external dependencies unless absolutely necessary.** Prefer stdlib Python, curl, and existing Hermes tools (`web_extract`, `terminal`, `read_file`).