fix(cli): add native FreeBSD self-executable path resolution

[PR9000]

What does this PR do?

Extracted from PR #1138 as requested during review.

cbm_detect_self_path had no branch for FreeBSD, falling through to the generic #else that reads /proc/self/exe. In this environment /proc is a native FreeBSD procfs mount left empty (only /compat/linux/proc is populated, for Linux-ABI processes under the compat layer), so the readlink always failed silently and the function fell back to the expected install path instead of the actual running binary's path.

This broke install: with no valid self path, the activation transaction could never confirm what to publish and aborted early before attempting any linkat/rename.

Add sysctl(CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1) for FreeBSD in src/cli/cli.c, matching the pattern already used in the test suites, and include <sys/sysctl.h> guarded by __FreeBSD__.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.