Draft AGENTS.md addition codifying autonomous PR-maintenance on Carson-owned approved-blocked PRs; surface to Carson for approval

N=2 empirical for autonomous merge-conflict resolution on PR #12919: fleet substrate resolved conflicts + pushed maintenance-only commits on 2026-07-06 (task ab1c2128, head 74527939) AND 2026-07-07 (task 1967c5af, head 3dae1a84 / 9f3c94d8). Both were merge-conflict-only, no substantive change, CI green, PR back to APPROVED/MERGEABLE/BLOCKED. Policy is currently emergent-not-stated. At lease: (1) draft a proposed AGENTS.md section titled "Autonomous PR maintenance on Carson-owned artifacts" specifying: (a) merge-conflict-only resolution commits are PERMITTED on PRs that are OPEN + APPROVED + previously MERGEABLE, provided the diff is conflict-resolution only (no substantive logic change), CI must go green post-push, no force-push, no re-review-required trigger; (b) FORBIDDEN: substantive edits, force-push, dismissing reviews, self-merge; (c) each maintenance push generates a fleet-task event referencing the PR and diff summary. (2) Write draft to memory/proposals/2026-07-agents-md-autonomous-pr-maintenance.md. (3) Surface the draft to Carson via one Slack DM in-thread on the existing #12919 thread (or via the next standup body) with the framing: "I have been doing X autonomously on #12919 — proposed AGENTS.md rule. Approve / modify / forbid." (4) Do NOT commit the AGENTS.md change until Carson approves. (5) If not surfaced by 2026-07-11T00:00Z, escalate priority and re-raise.