fix(probe-hook): harden probe capture file writes against symlink attacks

[gmoncor]

What: Port the hardening primitives already used by the production hook writer into bmad_loop_probe_hook.py's _atomic_write: a symlink/junction check, O_NOFOLLOW/dir_fd-anchored create+rename, an explicit 0o600 mode, and a short-write-safe write loop.

Why: The probe hook's _atomic_write currently does a plain open()+json.dump()+os.replace() with no symlink check and no explicit permissions, unlike the production relay's writer; SECURITY.md explicitly lists hook/signal-file handling as in-scope.

How:

• Follow the repo's existing twin-file pattern: duplicate the hardened write functions into bmad_loop_probe_hook.py rather than sharing an import.
• Add a parity/ablation test in tests/test_probe_hook.py mirroring tests/test_hook_script.py.

Testing: Run tests/test_probe_hook.py; new tests must fail against the pre-fix _atomic_write and pass after porting the hardening.

Changelog: "Security: hardened the probe-adapter capture writer against symlink-based file replacement, matching the production hook writer."

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