v0.24.1 — self-update fetches one at a time, and repairs a half-installed npx cache

Found while testing 0.24.0 end to end, minutes after it was released. Two npx installs of the same version ran at once on one machine and left half a package in npx's shared cache. Every later run of that version failed with ERR_MODULE_NOT_FOUND.

That was safe, because nothing restarts onto a version that fails its --version run, but the bridge stayed stuck. Several bridges on one machine told to update at the same moment is exactly that overlap.

• One fetch at a time per machine. The lock lives in ~/.cache/ai-bridge/, and only its holder releases it. A bridge waits up to 5 minutes for its turn; a lock older than 10 minutes counts as stale.
• Repair. When the --version run fails with a missing module inside an npx cache directory that holds the target version, that directory is removed and the fetch tried once more. A directory holding any other version is never touched.
• The fetch never throws. A lock or removal failure comes back as a failed fetch, and the updater retries it with the usual backoff.

0.24.0 bridges self-update to this as soon as a server asks for it.