feat: add codex-native plugin manifest + installer hook wiring

[Gradata]

Implements the GRA-93 follow-up from GRA-56 by making Codex a first-class install path in gradata-plugin.

What changed

• Added Codex-native manifest at .codex-plugin/plugin.json.
• Added portable plugin hooks manifest at hooks/hooks.json with node hooks/*.js commands.
• Extended setup/install.js with idempotent patchCodexConfig() to manage a Gradata block in ~/.codex/config.toml and wire lifecycle hooks (SessionStart, UserPromptSubmit, PostToolUse, PreCompact, Stop).
• Kept AGENTS.md patching path intact so Codex now triggers the same graduation + AGENTS maintenance lifecycle.
• Updated README supported-CLI docs for Codex install behavior.
• Added tests for Codex config creation and idempotent append behavior.

Verification

• node --check setup/install.js
• node --check setup/doctor.js
• node --test tests/install-idempotent.test.js (13/13 pass)

Notes

• Interactive end-to-end Codex smoke (two-session correction -> RULE -> AGENTS.md write) is not replayed in this PR because it requires a live Codex interactive run and daemon-state progression. This PR delivers the missing wiring needed for that smoke to pass.

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