Every other entry point takes the root lock #114

Blocked by #113. Adding the lock to paths that take none today is safe only once nothing nests the old lock.

Problem. These still bypass any lock:

• uninstall. Its only guard is findStone(), which doesn't see a download or a prepare in progress. Uninstalling in one window while another sets up leaves a half-installed root.
• "Reinstall Python" (reinstallGrail).
• gemdb.restartMcpServer.
• The gemdb.externalDatabase.* listener's stopMcpServer.
• The gemdb.mcp.readOnly toggle. Every open window's configuration listener restarts the router, so connected agents are dropped once per window.

Scope:

1. Run the bring-up entry points above through the lock.
   • For the toggle, record the router's mode in RouterState (readOnly; the field doesn't exist yet) and restart only when the recorded mode differs. A missing field counts as different, which costs one extra restart beside older versions.
   • reinstallGrail and restartMcpServer may be renamed or replaced by Grail and MCP versioning: gaps the new install mechanism must not inherit #110; check before starting.
2. uninstall takes withRootLock({contention:'skip'}). When the lock is held, refuse with: "Another window is setting up or starting GemDB. Try again when it finishes."
3. CHANGELOG entry.

Manual check:

• Toggle read-only with two windows open: one restart.
• Uninstall while another window is starting GemDB: refused.