feat: add contextual help and copy hash

[dinakars777]

Summary

git-time-machine now exposes its recovery controls through a contextual ? help overlay and lets users copy the selected full commit hash with y. The same slice broadens search beyond commit messages so users can filter by hash, author, and relative time while finding a recovery point.

Closes #18.

What Changed

• Added contextual help for timeline, diff, search, and restore flows.
• Added clipboard copy support with platform candidates for macOS, Windows, Wayland, and X11.
• Added status feedback when copying succeeds or clipboard support is unavailable.
• Expanded search matching from commit message only to message, hash, author, and relative time.
• Documented the GitHub research pass and updated README, roadmap, changelog, and landing-page controls.

Testing

• cargo fmt --check && cargo clippy -- -D warnings && cargo test
• cargo run -- --help
• git-time-machine --export-json against a temporary Git repository

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