feat: add @-triggered file path autocomplete to TUI input panel

[avoidwork]

Description

Add @-triggered file path autocomplete to the TUI input panel, allowing users to type @ followed by a filename fragment and select from a ranked list of matching files via recursive directory scan, with keyboard navigation and path insertion.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional change)
• Performance improvement
• CI / build / tooling

Testing

• Unit tests added/updated in tests/unit/tui.test.js for autocomplete state management
• CI test check passes (SUCCESS)
• CI lint check passes (SUCCESS)

Coverage

• Line coverage maintained

Checklist

• npm run lint passes
• Tests pass with maintained line coverage
• No forbidden patterns used
• Conventional Commit style applied

Files Changed

• src/tui/fileScanner.js — New: recursive filesystem scanner with .gitignore parsing, extension whitelist, symlink exclusion, 5000-entry cap
• src/tui/fileCache.js — New: TTL-based file list cache using tiny-lru
• src/tui/fileFilter.js — New: case-insensitive fuzzy matching with prefix scoring
• src/tui/autocompletePanel.js — New: scrollable overlay component with arrow key navigation and selection highlighting
• src/tui/inputPanel.js — Modified: @ character detection, cursor position tracking, autocomplete mode integration
• src/tui/app.js — Modified: autocomplete state management, keyboard navigation (up/down/enter/escape), path insertion
• src/config/schemas/tui.js — Modified: added fileCacheTtl, maxAutocompleteEntries, autocompleteMaxViewport, autocompleteBlacklist config options
• tests/unit/tui.test.js — Modified: added test coverage for autocomplete features
• openspec/specs/file-autocomplete/spec.md — New: full spec for file autocomplete capability
• openspec/specs/tui-event-routing/spec.md — Modified: added escape key override for autocomplete overlay
• openspec/specs/tui-interface/spec.md — Modified: added autocomplete overlay keyboard interaction requirements