feat(fuzzyfinder): add TODO/FIXME finder source

[jongio]

Adds a fuzzy finder source that scans the working tree for TODO, FIXME, HACK, BUG, and XXX markers so you can jump straight to the next thing that needs attention without leaving grut.

What it does

• Press T to open the finder over every marker in the tree.
• Each entry shows file:line plus the marker text (for example internal/app.go:42 TODO wire this up).
• Selecting an entry opens the file in the preview and scrolls to the exact line.

How it works

• New TodoSource in internal/panels/fuzzyfinder walks the tree reusing the same hidden-directory skip, non-navigable directory list, and .gitignore filtering the existing file source uses.
• Binary files are skipped (NUL byte sniff over the first 8KB), and file size and total result counts are capped so large repos stay responsive.
• Item gains a Line field so a selected entry can carry a jump target. FileSelectedMsg gains a matching Line, and the preview applies it once the file finishes loading (skipping binary and oversized files).
• T is bound to the new todo_finder action in the default, classic, and vim schemes, and the help overlay data (keybindings.json) plus generated docs/keybindings.md are updated.

Tests

• Marker scanning across all five markers, absolute-path values, binary skip, .gitignore respect, non-navigable dir skip, and nil/empty root.
• matchTodoLine table covering colon and no-colon messages, end-of-line markers, word-boundary rejection (autoTODOlist), and lowercase rejection.
• Preview jump-to-line after an async load, and a no-line case that stays at the top.
• Fuzzy finder selection emits a FileSelectedMsg carrying the line, and the todo_finder action opens the finder.

Closes #327

Co-authored-by: Copilot App [email protected]