Add CodeMirror editors to Memory, Skills, and Agents viewers

[aarlint]

Summary

• Replace read-only textContent rendering in Memory, Skills, and Agents viewers with full CodeMirror editors (matching the existing Plans viewer pattern)
• Add Save, Copy Content, and Copy Path buttons to each viewer header
• Add save-memory, save-skill, save-agent IPC handlers with path validation
• Fix hardcoded .claude string checks to use CLAUDE_DIRNAME constant derived from CLAUDE_DIR

Test plan

• Open a memory file — CodeMirror renders with syntax highlighting, editing works
• Open a skill — same editor behavior
• Open an agent — same editor behavior
• Save button persists changes to disk for all three viewers
• Copy Path / Copy Content buttons work
• Plan viewer still works unchanged
• Path validation rejects writes outside .claude directories

🤖 Generated with Claude Code

[abasiri]

Thank you. Will review and merge.

[aarlint]

Closing — this branch is identical to feature/project-groups (PR #3), which contains both the CodeMirror editor changes and the project groups feature. All changes will be merged via PR #3.