feat: make MCP workspaces request-stateless

[ejgallego]

This PR makes Lean Beam's local MCP application state request-stateless ahead of the 2026-07-28 wire-protocol update. Every workspace-bound tool now carries an explicit typed local workspace descriptor, so unrelated requests no longer depend on a mutable default workspace, startup root, Roots negotiation, or prior initialization call.

• Canonical roots lazily select private broker caches, and lean_drop_workspace is idempotent cache eviction that invalidates only that runtime’s handles.
• The current 2025-11-25 handshake remains in place; server/discover, per-request protocol metadata, modern result envelopes, and request-scoped logging remain follow-up work.
• This removes obsolete MCP init/list/root code, separates legacy protocol state from application cache state, closes descriptor schemas and decoding, and adds multi-root isolation and lifecycle regression coverage.