Agent panel overhaul, drag stability, session persistence fixes

[Anton-Horn]

Big batch of related work:

Agent panel

• Removed the marketplace. Settings view now only manages custom agents and prompts.
• Chat sessions persist across remounts (new agentSessionRegistry), so dragging an agent panel no longer loses the thread.
• New bundled cate-ask-user extension: the agent can ask the user a question and it renders as an inline card.

Drag and detach

• Cross-window terminal detach reworked: PTYs buffer with an explicit transfer target and abort timers, so detaching to a window that is still being created no longer drops output.
• Drag store tracks pendingDetach so the source panel doesn't flash during the IPC round-trip.
• Panel removal unified in removePanelFromWindow with transfer vs close semantics (renamed from removeDetachedPanelRecords).

Session persistence

• Autosave is gated while restore is hydrating, so an empty layout can't overwrite a saved one (Two state-persistence data-loss bugs: deferred workspace canvas wiped on activation + browser login lost on restart #220).
• Autosave now subscribes to every live canvas store in the workspace, not just the active one.
• projectWorkspaceStore recovers from .bak files and cleans up orphaned tmp files.

Misc

• Default panel sizes from user settings via resolvePanelSize.
• FS watcher uses the shared ignore matcher, drops the depth limit, stable subscriber ids.
• companion.ts had a literal NUL byte in a regex that made git treat the file as binary; now escaped as \x00.
• Lots of new tests (agent manager, shutdown, fs watch, terminal transfer, multi-canvas autosave, etc).

Full vitest suite passes (157 files, 1306 tests).