v0.2.3 — AO mechanism ports + attribution

Agent Orchestrator mechanism ports (#67) with explicit upstream attribution.

Added

• AO mechanism validators — nudge dedup (verify_nudge_dedup.py), stacked PR status (verify_stacked_pr.py), hook-signal health (verify_hook_signal.py), review supersede on sha-pin records; wired into validate-all.sh with FLEET_DISABLE_* kill switches.
• references/ao-adoptions.md — port map from Agent Orchestrator mechanisms to AF artifacts.
• ATTRIBUTIONS.md + NOTICE — credits for Agent Orchestrator (Copyright 2026 Untrivial, Apache 2.0).
• Example fixtures: .fleet/runs/example-fixture/nudge-state.json, pr-snapshot.json.
• Tests: 1368 total (new AO adoption test suites).

Changed

• engine.md AO MECHANISMS section; substrate-disable-knobs.md; Grok/Claude/Codex adapters declare activity_hooks: true.
• verify_sha_pin.py multi-file sha-pins/ layout + supersede enforcement.
• README Acknowledgments section pointing at attribution files.

Full changelog: CHANGELOG.md