feat: DEV-LEDGER.md as shared operating memory + session protocol

The ledger is the one-file source of truth for "what is currently
true" across Claude/Codex/human sessions:

- Orientation (live SHA, main tip, test count, harness state)
- Active Plan (currently Codex's 8-day extractor + harness plan
  with hard gates and fail-early thresholds)
- Open Review Findings (P1/P2, status)
- Recent Decisions (bounded to last 20)
- Session Log (bounded to last 20)
- Working Rules (no parallel work, branching rule, P1 block)

Narrative docs under docs/ sometimes lag reality; the ledger does
not. Every session MUST read it at start and append a Session Log
line before ending.

AGENTS.md: added a new "Session protocol" section at the top that
points at the ledger. Applies to any agent (Claude, Codex, future).

CLAUDE.md (new, project-local): project instructions for Claude
Code in this repo. Points at DEV-LEDGER.md and AGENTS.md, spells
out the deploy workflow and the Claude/Codex working model.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -1,5 +1,13 @@
 # AGENTS.md
 
+## Session protocol (read first, every session)
+
+**Before doing anything else, read `DEV-LEDGER.md` at the repo root.** It is the one-file source of truth for "what is currently true" — live SHA, active plan, open review findings, recent decisions. The narrative docs under `docs/` may lag; the ledger does not.
+
+**Before ending a session, append a Session Log line to `DEV-LEDGER.md`** with what you did and which commit range it covers, and bump the Orientation section if anything there changed.
+
+This rule applies equally to Claude, Codex, and any future agent working in this repo.
+
 ## Project role
 This repository is AtoCore, the runtime and machine-memory layer of the Ato ecosystem.