fix(cli): Always process @file references in interactive mode

[edlsh]

Summary

Fixes #14919

When users create prompts using the external editor (Ctrl+X), @file references were not being automatically loaded. Instead, the LLM had to initiate file reads itself, wasting tokens.

Root Cause

Interactive mode used isAtCommand() as a guard before calling handleAtCommand(). This heuristic (query.startsWith('@') || /\s@/.test(query)) failed in certain cases—especially for multi-line prompts from the external editor where @file might not be at the start or preceded by whitespace.

Non-interactive mode already calls handleAtCommand() unconditionally, so the fix is to match that behavior.

Changes

1. useGeminiStream.ts: Remove the isAtCommand() conditional guard—now always calls handleAtCommand() to match non-interactive behavior
2. atCommandProcessor.ts: Add a fast-path early exit (if (!query.includes('@'))) to avoid unnecessary parsing when no @ symbol exists
3. atCommandProcessor.test.ts: Add 3 new tests for edge cases:
   • Prompts without @ symbol (verifies fast path)
   • Email addresses (verifies they don't trigger file loading)
   • Multi-line prompts with @file on non-first line

Testing

• All 47 tests in atCommandProcessor.test.ts pass
• Full npm run preflight passes (build, typecheck, tests)
• Manual verification recommended with external editor workflow