AGENTS.md: stop the per-PR t27b remainder log; the gate enforces the shrink #8587

Part of epic #8326 (why the loop idles, how to speed up).

AGENTS.md's paragraph "The debt only shrinks" is a hand-kept log. Every t27b pull request appends its counts at the same place: 14 commits touched AGENTS.md between 2026-10-09 12Z and 2026-10-10 13Z. Each merge leaves every other open t27b lane DIRTY on that one paragraph, and lanes spend hours re-merging; #7905 and #7912 were re-merged several times for this alone.

Since #8236, check_budget() refuses a net gain of hand-written foreign lines, so the log enforces nothing. Replace it with the current measurement and the command that reproduces it, and drop the convention that a PR appends to it.

User Scenarios

• Given AGENTS.md contains a hand-kept log paragraph "The debt only shrinks" that every t27b PR must append to, When a contributor merges a t27b PR that updates that paragraph, Then all other open t27b lanes become DIRTY on that paragraph and must re-merge, costing hours.
• Given the check_budget() gate (since policy: hand-written code in any language but t27 may only shrink (Closes #8227) #8236) already refuses any net gain of hand-written foreign lines, When the manual log is replaced by a command that reproduces the current measurement, Then PRs no longer need to edit AGENTS.md for the log, eliminating the merge-conflict loop.

Requirements

• FR-001: The paragraph "The debt only shrinks" and its appended log lines MUST be removed from AGENTS.md.
• FR-002: AGENTS.md MUST include a command that, when executed, prints the current measurement (the same data the log recorded) and exits 0.
• FR-003: The convention that a t27b PR appends its counts to AGENTS.md MUST be dropped; no PR shall be required to edit AGENTS.md for this purpose.

Success Criteria

• grep -c "The debt only shrinks" AGENTS.md returns 0.
• The command embedded in AGENTS.md (e.g., python scripts/tri_loop/t27b.py --measure) runs and exits 0.
• git log --oneline --since="2026-10-09" --until="2026-10-11" -- AGENTS.md shows zero commits from t27b PRs after the change (i.e., no further log-appending commits).

Boundary

• AGENTS.md