fix: prevent containers from sticking during shutdown (#1826)

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## 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.

<sup>Written for commit 496180a706d2d3fb9799891c6a3891d473848f1a.
Summary will update on new commits.</sup>

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