docs(adapters): codex adapter docs reflect the tiered install

[plind-junior]

once the codex tier work lands, three doc surfaces go stale:

• adapters/README.md — the table still describes codex as "config.toml snippet": one file, one tier.
• adapters/codex/README.md — documents only the manual ~/.codex/config.toml edit. it should lead with vouch install-mcp codex, explain the project-local .codex/config.toml choice, describe what each tier adds (mcp wire / AGENTS.md snippet / prompts / capture), and keep the manual edit as the fallback path.
• docs/getting-started.md — mentions codex once, inside the mcp add one-liner; the install section should treat codex as a first-class host alongside claude-code.

no code. lowercase prose, match the existing adapter README voice. placeholder examples only — no real usernames, internal urls, or machine paths.

acceptance criteria

• adapters table row for codex lists the shipped tiers accurately
• adapters/codex/README.md covers installer-first flow, per-tier summary, the prompts-location decision from the T3 ticket, and the manual fallback
• getting-started shows the codex install alongside claude-code
• no stale references to the old copy-only behavior