afx spawn dup-guard substring-matches 'on it' anywhere in comment prose — match structured claims instead

[amrmelsayed]

Problem

The afx spawn duplicate-work guard treats ANY issue comment whose body contains the substring "on it" anywhere as a work claim. Verified at packages/codev/src/agent-farm/commands/spawn-worktree.ts:571-572:

const onItComments = issue.comments.filter((c) =>
  c.body.toLowerCase().includes('on it'),
);

Two false positives on one adopter workspace within two days, independently found by two lanes:

1. A guard refusal fired on an architect comment whose text read "...receipts before you spend ON IT" — ordinary prose; no assignee, no builder, no branch, no PR on the issue.
2. A refusal fired on the spawning architect's OWN takeover comment posted minutes earlier, which the guard read as a competing claim.

Both spawns required --force past a guard protecting nothing.

Why it matters beyond noise

A guard that cries wolf gets --forced reflexively, and then it is no guard at all on the day it is right. (--force also bypasses the dirty-worktree check, so reflexive forcing is doubly corrosive.)

Fix shape (hypothesis, for the builder to validate)

Match a STRUCTURED claim rather than prose. A claim is one of:

• an assignee on the issue,
• an open PR referencing the issue (the afx spawn: open-PR collision guard counts merged PRs since #1619 widened pr-search to --state all #1637-fixed open-state guard already computes this),
• a registered builder on the issue in the registry,
• at most: a comment that IS the claim — body (after trimming) STARTS with "on it" — never one that merely contains the substring.

Consider also exempting comments authored by the invoking identity (a self-claim is not a collision).

Keep in the fix: the refusal path's semantics are correct

Both field refusals half-created NOTHING — no worktree, no branch, no registration. That fail-closed behavior is the standard the crash-mid-spawn path in #1712 needs to match; do not regress it while reworking the matcher.