feat(skills): show runtime-discovered skills in /memory-skills modal

[chandra447]

Summary

• include skills currently loaded by Pi in this session (not only extension-managed roots)
• merge runtime-loaded skills with managed skills in /memory-skills
• mark external skills as read-only and show path info in modal
• add in-modal category filters (f panel) and runtime-discovery tests
• move extension-owned storage root to ~/.pi/agent/pi-hermes-memory
• move extension-managed global skills to ~/.pi/agent/pi-hermes-memory/skills
• add startup migration from legacy ~/.pi/agent/memory into ~/.pi/agent/pi-hermes-memory
• normalize legacy flat skills/*.md into skills/<slug>/SKILL.md to avoid Pi skill-name conflicts

Why

This aligns /memory-skills with what Pi actually has loaded in the current session, keeps external skills safe (read-only), and avoids polluting the shared ~/.pi/agent/skills directory by using an extension-owned storage root.

Migration behavior

• Default writes now target ~/.pi/agent/pi-hermes-memory
• Existing legacy assets in ~/.pi/agent/memory are migrated on startup
• Existing flat markdown skill files under ~/.pi/agent/pi-hermes-memory/skills/*.md are auto-converted into Pi-compatible folder skills (<slug>/SKILL.md)

Validation

• npm run check
• npm test (full suite)