feat(adapters): codex T2 — AGENTS.md fenced snippet

[plind-junior]

the codex adapter stops at T1 (mcp wire). codex reads AGENTS.md for project instructions the same way cursor does — and the cursor adapter already ships this exact tier (adapters/cursor/install.yaml T2: AGENTS.md.snippet with fenced_append: true). without it, a codex session gets the kb tools but no standing instructions on when to recall or how to propose through the review gate.

proposed change

• add adapters/codex/AGENTS.md.snippet — same content contract as cursor's snippet (what vouch is, recall-before-answering, propose-don't-write, pointer to vouch review), phrased host-neutrally so the two snippets stay in sync.
• add a T2 tier to adapters/codex/install.yaml with the standard fence markers (<!-- BEGIN vouch --> / <!-- END vouch -->), fenced_append: true, dst AGENTS.md.
• _install_fenced in src/vouch/install_adapter.py already handles create-vs-append and idempotence; no installer changes needed.

acceptance criteria

• installing at T2 appends the fenced snippet to an existing AGENTS.md and creates the file when absent
• re-running leaves the file unchanged; an edited fence body is replaced within the markers
• snippet guidance matches the cursor snippet's invariants (recall first, all writes via proposals, review stays human)
• covered in tests/test_install_adapter.py