feat(adapters): codex T3 — custom prompts mirroring the vouch slash commands

[plind-junior]

claude-code T3 ships nine slash commands (/vouch-recall, /vouch-status, /vouch-resolve-issue, /vouch-propose-from-pr, plus the company-brain set — see adapters/claude-code/install.yaml). codex has an equivalent surface: custom prompts — markdown files codex exposes as slash commands. codex users currently get none of them, so the guided flows (recall before answering, distill a merged pr into proposals) are claude-code-only.

proposed change

• mirror the claude-code command set as codex custom prompts under adapters/codex/prompts/<name>.md, installed as a T3 tier in adapters/codex/install.yaml.
• scope decision to resolve first: codex historically reads custom prompts from ~/.codex/prompts/ (user-global). the vouch install-mcp <host> — one-command adapter writer #179 rule is that a project-scoped install never touches home-directory state. part of this ticket is checking the current codex cli for a project-local prompts location; if one exists, ship T3 there. if prompts are still global-only, T3 must not silently write to ~ — either gate it behind an explicit opt-in flag or ship the prompt files in the adapter dir with README instructions, and record the choice in the manifest comment.
• keep prompt bodies in lockstep with adapters/claude-code/.claude/commands/*.md — same pattern as the openclaw skills mirror, enforced by a sync test (tests/test_openclaw_plugin_manifest.py is the precedent for body-equality checks).

acceptance criteria

• the four core commands (recall / status / resolve-issue / propose-from-pr) available as codex prompts; company-brain set included if the mirroring stays mechanical
• no home-directory writes from a default project-scoped install
• a sync test asserts codex prompt bodies match the corresponding claude-code command bodies
• adapters/codex/README.md documents how the prompts surface inside codex