🖱️ Mouse support for interactive TUI (EPIC)

[shouze]

Context

The interactive TUI (src/tui.ts) is keyboard-only. Users must use arrow keys and shortcuts to fold/unfold a repo's extracts (▸/▾), toggle selection (✓) on repos/extracts, and page through long result lists (Page Up/Down, Ctrl+U/D). Terminal mouse reporting (xterm/SGR protocol) is a standard, dependency-free mechanism that would let users click the fold arrow, click a checkbox, or scroll with the wheel — closer to a GUI interaction model and more discoverable for new users.

Solution

Add mouse support to the TUI using the standard SGR mouse-reporting protocol (\x1b[?1000h\x1b[?1006h), parsed on the same raw stdin channel already used for keyboard input in runInteractive(). No new dependency is introduced (picocolors remains the only styling dependency per AGENTS.md).

Sub-issue	Feature	Scope
Mouse tracking foundation	Enable/disable protocol + escape-sequence parsing pipeline	Infrastructure, blocks the two below
Click-to-toggle	Fold/unfold (▸/▾) and select/deselect (✓) on repo/extract rows	MVP interaction
Scroll-wheel support	Wheel-up/down mapped to existing scroll logic	Depends on foundation
UI consistency	Dimmed/inactive ✓ instead of blank space for deselected checkboxes	Independent, can ship first

Acceptance Criteria

• All sub-issues are closed and merged.
• Clicking the fold arrow, clicking a checkbox, and using the scroll wheel all work in at least one common terminal (iTerm2 or Terminal.app on macOS, plus one Linux terminal noted in manual testing).
• No new runtime dependency was added.
• Mouse tracking is reliably disabled on exit (including Ctrl+C) — verified manually that the terminal doesn't stay in mouse-reporting mode after the process exits.
• Keyboard-only usage is fully unaffected (regression-free).

Definition of Done

• bun test passes.
• bun run lint passes.
• bun run format:check passes.
• bun run knip passes (no unused exports/imports).
• bun run build.ts compiles successfully.
• New pure logic (hit-testing, column detection, escape-sequence parsing helpers) lives in a testable module with a companion *.test.ts, per the pure-function-first architecture in AGENTS.md.
• Documentation updated: docs/reference/keyboard-shortcuts.md documents mouse interactions; README.md interactive-mode section mentions mouse support if applicable.
• Changelog / release notes entry prepared per AGENTS.md § Release process (minor release).