docs: update IMPLEMENTATION.md with v0.3.0 detailed plan and GitHub issue cross-references

- Add detailed v0.3.0 plan with Issue #48 (race condition fix) as priority
- Expand Issue #55 with phased approach for UX improvements
- Add Issue #47 (multi-root workspaces) as optional feature
- Add comprehensive development tasks breakdown for v0.3.0
- Update v0.4.0 with security/performance issues (#51, #52, #54)
- Add GitHub issue cross-references throughout
- Set realistic timelines (Q1 2026 for v0.3.0, Q2 2026 for v0.4.0)
- Update v1.0.0 with Issue #53 (accessibility)
- Remove duplicate development tasks sections

This aligns IMPLEMENTATION.md with current open issues and provides clear
roadmap for next releases.

Author: jcouball