feat(mcp): read discovered skill files remotely

[eshwar-sundar-glean]

Summary

• Move discovered skill file reads to the remote read_skill_files MCP tool.
• Return the server-provided lazy skill index without materializing skill files on disk.
• Keep a process-local tool metadata cache for HITL approval and file_args schema handling.
• Preserve compatibility with older servers that still return the full legacy JSON response by keeping that response in memory only.
• Add read_skill_files to the policy-controlled MCP surface and update the glean_run instructions for Claude Code, Cursor, and Codex.
• Remove the local skill-tree writer, stale skill eviction, and SKILLS_BASE_DIR launcher setup.

Validation

• npm run typecheck:bundle
• npm run test:bundle (322 tests passed)
• npm run build