fix: prioritize @ file search over slash command completion

[Br1an67]

TLDR

Fix @ file search not working after selecting a custom slash command on the same line.

Fixes #2518

Dive Deeper

Root Cause

In useCommandCompletion, the completion mode detection checks for slash commands before checking for @ file references:

if (cursorRow === 0 && isSlashCommand(currentLine.trim())) {
  return { completionMode: CompletionMode.SLASH, ... };
}
// @ detection never reached when line starts with /

Since isSlashCommand() returns true for any line starting with / (excluding // and /*), typing @ after a completed slash command like /qc:create-issue @readme still matches the slash command check first, and the @ file search code is never executed.

Fix

Reorder the completion mode detection to check for @ file references before slash commands. When the cursor is positioned after an @ character (within the current word boundary), AT mode takes priority. If no @ is found, the slash command check runs as before.

This preserves all existing behaviors:

• /help → SLASH mode (no @ in text) ✓
• /model qwen3 → SLASH mode (no @ in text) ✓
• /qc:create-issue @readme → AT mode (@ found before cursor) ✓
• @file → AT mode (no change) ✓
• plain text → IDLE mode (no change) ✓

Testing Matrix

• All 15 existing + new tests pass (useCommandCompletion.test.ts)
• Added regression test: @ after slash command triggers AT completion
• Pre-commit hooks (prettier + eslint) pass

[tanzhenxin]

@Br1an67 Thanks for your contributions to Qwen Code! But we are really appreciate if you can take time to do manual testing before submitting PR, I mean it is kind of the bottleneck work of current "Vibe Coding" world.

A screenshot after fix would be very great. We will take it as a proof of work to guide us to allocate resources for the PR review.