feat(supervisor): expose concurrent UDS auto-start

[kvinwang]

Summary

Expose the Supervisor client's existing Unix-socket auto-start flow through the CLI and make concurrent starters converge on one daemon.

Changes

• Add CLI options for auto-starting a missing Supervisor over a Unix socket.
• Serialize concurrent starts with an owner-only lock and re-probe after acquiring it.
• Start the Supervisor with umask 077, so its runtime files and socket are not group/world writable.
• Keep structured command output on stdout by sending tracing output to stderr.
• Return the shutdown response before notifying Rocket to stop.

This branch is rebased directly onto master. It does not include the client-side socket path validation from the closed PR #893; normal Unix socket and parent-directory permissions remain the access-control boundary.

Verification

• cargo check --manifest-path dstack/Cargo.toml -p supervisor-client -p supervisor
• cargo test --manifest-path dstack/Cargo.toml -p supervisor-client -p supervisor
• cargo fmt --manifest-path dstack/Cargo.toml --all
• git diff --check

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[kvinwang]

Closing the concurrent UDS auto-start feature because the supported deployment has one VMM owner per runtime directory. The two independent fixes were split into #994 (machine-readable client output) and #995 (graceful shutdown response). PR #841 was updated to test the client lifecycle directly without requiring CLI auto-start or concurrent-starter behavior.