docs(adr): record check precision defaults in an ADR that amends ADR 0003 #8537

Refs: #7572

Agent Brief

Type: Task
Summary: Write one new ADR that amends ADR 0003 with the precision defaults for checks: zero effective false positives for a blocking check, under 10% for an advisory one, plus the exception rules.

Current behavior:

• docs/adr/0003-verification-guards-earn-default-on-by-measured-precision.md says a guard earns default-on by measured precision but states no number; its reference guard shipped at 57% precision (docs/adr/0003-verification-guards-earn-default-on-by-measured-precision.md:48-49).
• No ADR carries an Amends: line for ADR 0003 on main. The newest ADR is docs/adr/0079-dispatch-tagged-phase-reviews-to-quality-gate-at-the-phase-boundary.md.
• House style for the line is - Amends: [ADR 0049](0049-run-ci-lanes-on-github-hosted-runners-under-trigger-and-token-hardening.md), in the header list (docs/adr/0074-review-risky-dependabot-updates-in-a-model-job-and-act-in-a-scripted-job.md:7).
• feedback.disable_not_useful_ratio and exceptions.max_days exist only in the decision record: no file under docs, plugins or scripts mentions disable_not_useful or max_days.

Desired behavior:

• A new docs/adr/NNNN-<kebab-title>.md, numbered one above the highest on main when the lane runs, in the shape of docs/adr/0079-dispatch-tagged-phase-reviews-to-quality-gate-at-the-phase-boundary.md (title, - Status: accepted, - Date:, Context, Decision, Consequences), written the way /architecture:record-decision writes one.
• Its header list carries - Amends: [ADR 0003](0003-verification-guards-earn-default-on-by-measured-precision.md).
• Decision text:
  • A blocking check ships with 0 effective false positives on its corpus.
  • An advisory check stays under 10% false positives; the 10% is named as a corpus-precision proxy, not a measured per-guard rate.
  • feedback.disable_not_useful_ratio is not adopted until guardrails: grade every default-on guard's precision from the per-session hook event log (ADR 0003) #3756 produces a per-guard signal.
  • Exceptions expire after 90 days by default, as a team setting.
  • Break glass is an inline exception that carries a reason, an expiry and a person's approval.
  • Existing guards keep their recorded justification; nothing is re-graded by this ADR.
• Consequences name the enforcement ladder's guardrail criterion (docs/conventions/enforcement-ladder/README.md) as the reader of these numbers, and guardrails: grade every default-on guard's precision from the per-session hook event log (ADR 0003) #3756 as the open work that grades guards.

Key interfaces:

• docs/adr/0003-verification-guards-earn-default-on-by-measured-precision.md (read; not edited).
• scripts/check-adr-numbers.sh (the number must not collide).

Acceptance criteria:

• Exactly one file under docs/adr has an Amends: line naming ADR 0003, the new ADR.
• The new ADR contains 0 effective false positives, 10%, 90 days and #3756.
• The PR's CI runs scripts/check-adr-numbers.sh --check (lint-repo) and it passes.
• The diff touches only docs/adr/. The PR's CI checks changelog fragments and passes with no fragment, because no plugin's shipped files change.
• CI's lint-repo job (which runs check-adr-numbers.sh and the markdown lint) passes.

Out of scope:

• Editing ADR 0003 itself or any guard.
• Adding the 90-day exception setting to a plugin; this ADR records the default only.
• The ladder's guardrail criterion text (sibling slice ladder-v2).

Origin:

• 0 effective false positives blocking, under 10% advisory: ours, from Google's published bar, D31, Q199, D84.
• 10% named as a corpus-precision proxy: ours, D116, Q559.
• Drop feedback.disable_not_useful_ratio until guardrails: grade every default-on guard's precision from the per-session hook event log (ADR 0003) #3756: owner's, D84.
• Exception expiry 90 days as judgment inside the team setting: owner's, D84.
• Break glass inline exception with reason, expiry and approval: hers (Lauren's), D64.

Do not run with: none. No other open slice edits docs/adr/ for ADR 0003; two ADR-writing PRs racing for one number is caught by check-adr-numbers.sh.

Slice of #7572, drafted by the Issue Triage session from the thread's recorded decisions.