fix(gate): cancel foreground producer process trees

[screenleon]

Summary

• persist verified producer process identity on parent operations
• stop and reap the active foreground preflight or detached supervisor process tree before cancelling recorded child runs
• keep incomplete or unverifiable termination fail-closed as indeterminate
• isolate nested preflight operation ownership and synchronize gate cancel command documentation
• add regression coverage for foreground preflight, detached supervisor, identity mismatch, already-exited producers, and repeated cancellation

Why

pmctl gate cancel previously traversed only recorded reviewer child runs. During foreground preflight, the parent operation already existed but no reviewer child had been attached, so cancellation could terminalize the operation while the preflight producer process tree continued running.

Verification

• targeted remediation gate: Final: GO; all five reviewers passed; operation op-20260728T072646Z-01ef21 completed with 0 unresolved children
• gate result: /home/screenleon/.local/share/pm-dispatch/state/projects/4633b7e7f780014195b603f84ce281c3a1afd97b/runs/gate-20260728-162646-1419363/.gate-results/gate-20260728-162657.md
• authoritative full suite: 97 passed, 0 failed, 0 skipped
• full result: .pm-dispatch/test-results/latest-full.json
• canonical current-tree verification: PASS for tree d9759688023394fa8417a8f4e83093e147d3097925c0fab00014f164b5a10197
• directly affected suites: test-pmctl-gate 44/44, test-pmctl-operation 17/17, test-pr-gate 194/194, test-core-schemas 73/73