revert: remove @-triggered file path autocomplete (not ready)

[avoidwork]

Description

Reverts PR #793 — the @-triggered file path autocomplete feature is not ready for merge.

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 changes)
• Performance improvement
• CI / build / tooling

Testing

• Revert commit removes all 18 files introduced by feat: add @-triggered file path autocomplete to TUI input panel #793
• No code logic changes — pure revert

Coverage

• Line coverage maintained (reverted files removed)

Checklist

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