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)

skills/apple/apple-notes/SKILL.md

@@ -0,0 +1,88 @@
+---
+name: apple-notes
+description: Manage Apple Notes via the memo CLI on macOS (create, view, search, edit).
+version: 1.0.0
+author: Hermes Agent
+license: MIT
+platforms: [macos]
+metadata:
+  hermes:
+    tags: [Notes, Apple, macOS, note-taking]
+    related_skills: [obsidian]
+---
+
+# Apple Notes
+
+Use `memo` to manage Apple Notes directly from the terminal. Notes sync across all Apple devices via iCloud.
+
+## Prerequisites
+
+- **macOS** with Notes.app
+- Install: `brew tap antoniorodr/memo && brew install antoniorodr/memo/memo`
+- Grant Automation access to Notes.app when prompted (System Settings → Privacy → Automation)
+
+## When to Use
+
+- User asks to create, view, or search Apple Notes
+- Saving information to Notes.app for cross-device access
+- Organizing notes into folders
+- Exporting notes to Markdown/HTML
+
+## When NOT to Use
+
+- Obsidian vault management → use the `obsidian` skill
+- Bear Notes → separate app (not supported here)
+- Quick agent-only notes → use the `memory` tool instead
+
+## Quick Reference
+
+### View Notes
+
+```bash
+memo notes                        # List all notes
+memo notes -f "Folder Name"       # Filter by folder
+memo notes -s "query"             # Search notes (fuzzy)
+```
+
+### Create Notes
+
+```bash
+memo notes -a                     # Interactive editor
+memo notes -a "Note Title"        # Quick add with title
+```
+
+### Edit Notes
+
+```bash
+memo notes -e                     # Interactive selection to edit
+```
+
+### Delete Notes
+
+```bash
+memo notes -d                     # Interactive selection to delete
+```
+
+### Move Notes
+
+```bash
+memo notes -m                     # Move note to folder (interactive)
+```
+
+### Export Notes
+
+```bash
+memo notes -ex                    # Export to HTML/Markdown
+```
+
+## Limitations
+
+- Cannot edit notes containing images or attachments
+- Interactive prompts require terminal access (use pty=true if needed)
+- macOS only — requires Apple Notes.app
+
+## Rules
+
+1. Prefer Apple Notes when user wants cross-device sync (iPhone/iPad/Mac)
+2. Use the `memory` tool for agent-internal notes that don't need to sync
+3. Use the `obsidian` skill for Markdown-native knowledge management