fix(hook_check): skip hook warning on Windows when --claude-md mode is installed

[SrHuevo]

Summary

• On Windows, rtk init -g cannot install a PreToolUse hook — it falls back to --claude-md mode and writes the RTK instructions block into ~/.claude/CLAUDE.md. That's the only valid integration on this platform.
• But status() still returns Missing in that configuration, so maybe_warn() keeps printing /!\ No hook installed — run `rtk init -g` for automatic token savings every 24h.
• The advice is impossible to follow: running rtk init -g on Windows just re-emits the --claude-md fallback message (Hook-based mode requires Unix (macOS/Linux). Windows: use --claude-md mode). The user is nagged forever about an integration they can never install.
• This PR teaches status() to recognize the --claude-md integration on Windows by looking for the <!-- rtk-instructions marker in ~/.claude/CLAUDE.md, and return HookStatus::Ok in that case.
• The check is narrowly gated on cfg!(windows) to stay minimal and avoid reopening the broader "hook masking" discussion from d85fe33 — on macOS/Linux, behavior is unchanged.

Reproduction (before this patch, on Windows)

> rtk init -g --no-patch
[rtk] Hook-based mode requires Unix (macOS/Linux).
       Windows: use --claude-md mode (automatic fallback).
> rtk init -g --claude-md
[rtk] Installed RTK instructions into ~/.claude/CLAUDE.md
> rtk git status
[rtk] /!\ No hook installed — run `rtk init -g` for automatic token savings
...

After the patch, once <!-- rtk-instructions is present in ~/.claude/CLAUDE.md, the warning stops — on Windows only.

Test plan

• New unit tests cover the three shapes of claude_md_mode_installed: marker present, marker absent, file missing.
• test_status_returns_valid_variant extended with a Windows + --claude-md branch asserting HookStatus::Ok.
• cargo fmt --all && cargo clippy --all-targets && cargo test — I don't have a Rust toolchain on the Windows machine where I reproduced this, so relying on CI for the pre-commit gate.
• Manual testing on Windows: confirmed the warning fires in v0.35.0 with ~/.claude/CLAUDE.md containing the RTK block; after this patch it should go silent.

Targets develop per CONTRIBUTING.md.

[CLAassistant]

All committers have signed the CLA.