feat: add export_report tool for markdown session reports

[TerminalGravity]

Closes #5

Adds export_report MCP tool that generates markdown session reports from timeline data.

Features:

• Summary stats table (prompts, corrections, commits, tool calls, errors, compactions)
• Tool usage breakdown (top 10 most-used tools)
• Daily activity grouped by day with commits, errors, and corrections highlighted
• Prompt quality trend table showing daily correction rates over time
• Configurable period (day/week/month/quarter) or custom date range via since/until
• Optional saveTo parameter to write report to file
• Scope filtering (current/related/all projects)

Builds on existing timeline infrastructure — reuses getTimeline, listIndexedProjects, and getRelatedProjects.

[TerminalGravity]

Superseded by new PR