docs: publish project roadmap

[dinakars777]

Summary

This publishes a focused roadmap that makes git-time-machine's next work visible without turning the issue tracker into a broad backlog. It frames near-term recovery improvements, recovery workflow ideas, distribution plans, and explicit non-goals so future PRs can stay small and testable.

Details

• Add ROADMAP.md with product principles, current baseline, near-term work, recovery workflows, distribution plans, and issue policy.
• Link the README contributing section to the roadmap while keeping a short active-consideration list.
• Update the Unreleased changelog.

Testing

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo build
• git diff --check