fix(hook): hook warning repeats on every command on Windows

[aimardcr]

Summary

• std::fs::write(path, b"") writing 0 bytes to an already-empty marker file does not update the mtime on Windows
• The rate-limiting check uses meta.modified() so the marker always looked stale, causing the "No hook installed" warning to fire on every invocation instead of once per day
• Write a non-empty payload (b"1") so Windows always updates the mtime

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test — 982 tests pass
• Manual testing: rtk output inspected
  • Set marker mtime to 5 days ago → rtk git status — warning shown (expected, marker expired)
  • rtk git status again — warning suppressed (rate-limiting works)
  • rtk tree — warning suppressed (persists across commands)

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