Move privileged worker probe to entrypoint

[cbusillo]

Summary

• run the privileged-operation schema probe in the container entrypoint before the worker Python process starts
• enforce a fixed 90-second GNU timeout with fail-closed timeout, killed, and probe-failure telemetry
• isolate the probe from unrelated privileged worker environment values
• pass one-time completion evidence through an inherited, unlinked file descriptor
• refuse direct continuous-worker startup without that supervised evidence
• remove the in-process Python subprocess path that could block before its timeout began

Validation

• uv run --extra dev launchplane ci unittest-shard local (3,080 targets)
• uv run --extra dev mypy control_plane tests
• focused Ruff and format checks
• ShellCheck and shfmt
• production container build
• real container + PostgreSQL smoke emitted startup, schema probe, store initialized, first poll, and poll succeeded
• two independent final reviews: approved

Refs #2219