fix(hooks/codex): write RTK reference to AGENTS.override.md, not AGENTS.md

[YOMXXX]

Summary

`rtk init --codex` previously injected its `@RTK.md` reference into Codex's base `AGENTS.md` file, mingling RTK output with user-authored base instructions. Codex's documented layering puts user/tool overrides in `AGENTS.override.md` — the least-destructive layer — so this PR routes RTK there. Existing installs that wrote into `AGENTS.md` are migrated cleanly: the legacy RTK reference (and any older inline `` block) is stripped from `AGENTS.md`, and the modern `@RTK.md` reference is added to `AGENTS.override.md`.

Reproduction

```bash
mkdir -p /tmp/codex-repro && cd /tmp/codex-repro
printf '# Base rules\nAnswer in Spanish.\n' > AGENTS.md
rtk init --codex
diff -u <(printf '# Base rules\nAnswer in Spanish.\n') AGENTS.md

Before this PR: diff (AGENTS.md was modified, @RTK.md appended)

After this PR: no diff (AGENTS.md untouched, @RTK.md lives in AGENTS.override.md)

cat AGENTS.override.md

After this PR: contains the @RTK.md reference

```

Root cause

`src/hooks/init.rs:2287-2353` (`run_codex_mode` / `run_codex_mode_with_paths`): both global and local codex install paths joined `codex_dir / AGENTS.md` and called `patch_agents_md()` on that path. `hook_check.rs:172` similarly used `AGENTS.md` as the install marker.

Fix approach

• Add `CODEX_OVERRIDE_MD = "AGENTS.override.md"` constant.
• Replace `run_codex_mode_with_paths(agents_md_path, rtk_md_path, …)` with `run_codex_mode_at(codex_dir, global, ctx)`. Inside, derive `AGENTS.md` (read-only legacy migration target) and `AGENTS.override.md` (write target) from `codex_dir`.
• New `migrate_agents_md_to_override(agents_md, _override, ctx)`: strips RTK-owned content (`@RTK.md` reference and/or rtk-instructions marker block) from a legacy `AGENTS.md`. The companion `patch_agents_md(&override_md_path, …)` call (already idempotent) then writes the canonical `@RTK.md` reference into `AGENTS.override.md`. The legacy inline block content is not copied to override.md — it would be stale relative to the freshly written `RTK.md` and `patch_agents_md` would treat it as a migration candidate anyway.
• `uninstall_codex_at` now scrubs both `AGENTS.md` and `AGENTS.override.md`, so users upgrading from an older RTK install get fully cleaned up regardless of which file was touched.
• `hook_check.rs:172` now treats either `AGENTS.md` or `AGENTS.override.md` as a valid codex-installed marker.
• `run_codex_mode_with_paths(…)` is retained behind `#[cfg(test)]` as a thin compatibility shim so unchanged tests keep compiling.

Behavior changes

• Fresh install (no legacy state): writes `AGENTS.override.md` and `RTK.md`; `AGENTS.md` is never touched.
• Legacy install (RTK content lives in `AGENTS.md`): on next `rtk init --codex`, RTK content is stripped from `AGENTS.md` (user content preserved verbatim) and re-added to `AGENTS.override.md`.
• Uninstall: scrubs both files (legacy + current).
• Public API (`pub fn run_codex_mode`) unchanged.

Test plan

• `test_run_codex_mode_global_writes_absolute_reference_to_override_md` — fresh install path writes `AGENTS.override.md` only, `AGENTS.md` not created.
• `test_run_codex_mode_preserves_user_agents_md_content` — pre-existing `AGENTS.md` byte-for-byte unchanged.
• `test_run_codex_mode_migrates_legacy_agents_md_reference` — legacy `@RTK.md` reference moved out of `AGENTS.md` into `AGENTS.override.md`.
• `test_run_codex_mode_migrates_legacy_agents_md_rtk_block` — legacy inline `rtk-instructions` block stripped from `AGENTS.md`; `AGENTS.override.md` holds the modern reference.
• `test_uninstall_codex_at_cleans_legacy_agents_md_and_override` — uninstall touches both files.
• Existing codex tests updated (one renamed) and pass unchanged.
• `cargo fmt --all` clean.
• `cargo clippy --all-targets` zero warnings.
• `cargo test --bin rtk -- --test-threads=8` 1905 passed, 0 failed.

Fixes #1943

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