docs: engine contract surfaces + full unification plan (phase 0)

Documents the engine contract (COVEN.md section + AGENTS.md rule line) pointing at coven/docs/ENGINE-CONTRACT.md (contract_version 1), and adds the full 6-phase coven CLI unification plan. Companion to OpenCoven/coven#346; the contract doc link goes live when that lands. Docs-only, no behavior change.

Author: BunsDev

AGENTS.md

@@ -26,6 +26,7 @@ Agent-facing rules for working on Coven Code. Mirrors and extends `src-rust/.cla
 - NEVER modify generated files directly. Generated artefacts in this repo:
   - `src-rust/Cargo.lock` — regenerated by cargo; for version bumps use [`scripts/bump-version.py`](scripts/bump-version.py).
   - `npm/package.json` `version` field — also stamped by `bump-version.py`.
+- Contract surfaces (see COVEN.md "Engine contract") are semver-guarded — breaking them requires a contract version bump.
 
 ## Commands
 

COVEN.md

@@ -100,6 +100,17 @@ Add Coven-specific tools (e.g. `coven_session_tool.rs`) and register in `tools/s
 
 ---
 
+## Engine contract
+
+coven-code is the engine behind the unified `coven` CLI. The exact CLI/env/
+stream surfaces coven depends on are specified in
+[coven/docs/ENGINE-CONTRACT.md](https://github.com/OpenCoven/coven/blob/main/docs/ENGINE-CONTRACT.md)
+(`contract_version: 1`). Do not change flags, output formats, or exit codes
+listed there without bumping the contract version and coordinating a coven
+`engine.lock` update.
+
+---
+
 ## Release checklist
 
 When cutting a `coven-code` release: