feat(tui): @-triggered fuzzy file picker in input

[bug-ops]

Summary

Add @-triggered fuzzy file search popup to the TUI input area. Typing @ in Insert mode opens an interactive file picker that fuzzy-matches project files by name, path, and extension.

Motivation

When composing messages in TUI, users often need to reference files. Currently they must type full paths manually. An @-triggered picker (similar to VS Code, GitHub, etc.) provides fast file discovery without leaving the input.

Design

UX Flow

1. Type @ in Insert mode → popup opens above input
2. Continue typing to fuzzy-filter (e.g., @main.rs, @src/app)
3. Up/Down navigate results (top 10 shown)
4. Enter/Tab accept → replaces @query with relative file path
5. Esc dismisses without inserting
6. Backspace past @ auto-dismisses

Implementation

• Fuzzy engine: nucleo-matcher (same as Helix editor — fast, zero-alloc)
• File walking: ignore crate (already in workspace, respects .gitignore)
• File index: cached with 30s TTL, rebuilt on next @ trigger
• Rendering: overlay popup above input (reuses Clear + centered_rect pattern from confirm modal)
• State: FilePickerState { query, matches, selected, anchor } in App

New files

• crates/zeph-tui/src/file_search.rs — file index + fuzzy matching
• crates/zeph-tui/src/widgets/file_picker.rs — popup widget

Modified files

• crates/zeph-tui/src/app.rs — state + key interception
• crates/zeph-tui/src/widgets/mod.rs — module export
• crates/zeph-tui/Cargo.toml — add nucleo-matcher, ignore

Detailed plan

.local/plan/tui-file-picker.md

Acceptance Criteria

• @ in Insert mode opens file picker popup
• Fuzzy search matches file name, directory, and extension
• Results update on each keystroke
• Enter/Tab inserts relative path at cursor position
• Esc dismisses without side effects
• .gitignored files excluded
• Snapshot tests for widget rendering
• Unit tests for fuzzy matching logic