feat: add export_timeline tool for markdown session reports

[TerminalGravity]

Closes #5

Adds a new export_timeline MCP tool that generates structured markdown reports from timeline data:

• Summary stats: total events, active days, avg events/day, commits, corrections, errors
• Activity breakdown by event type
• Daily activity with visual bars
• Corrections, commits, errors sections with details
• Configurable period: day/week/month/quarter
• Author filtering for commits
• Relative date support (7days, 2weeks, etc.)

Also adds memory/ to .gitignore.

All 44 tests pass.

[TerminalGravity]

Reviewed the export_timeline tool. README additions (troubleshooting, CLAUDE_PROJECT_DIR docs, Node 20+ badge fix) are solid improvements. The memory/ gitignore addition makes sense. Looks good to merge — note this overlaps with #134, so merge one and close the other to avoid conflicts.

[TerminalGravity]

Export timeline is a nice addition — addresses issue #5 directly. The markdown output format looks clean and the tool registration follows the existing patterns well.