fix: prevent containers from sticking during shutdown

[luke-lombardi]

Summary by cubic

Fixes containers sticking during shutdown by reliably cancelling startup when a stop is issued and recovering from wedged workspace mounts (GeeseFS) after SIGKILL.

• Bug Fixes

  • Replace “build” cancels with per-container cancels; cancel startup contexts on stop events and after worker reconnects.
  • Close startup race: cancelContainerIfAlreadyStopping checks local lifecycle state first, then repository with a 2s timeout.
  • Recover stuck shutdowns: abortStuckWorkspaceMount unmounts GeeseFS workspaces after SIGKILL timeout even if no runtime existed, without impacting other containers.
  • Swap cancelBuild → cancelContainer in lifecycle handling; improved logging and tests for race and mount recovery.

• Dependencies

  • Update github.com/beam-cloud/geesefs replacement to a newer commit.

^{Written for commit 496180a. Summary will update on new commits.}