fix: keep @ file references available in /skill argument text

[ShreyanshVaibhaw]

Summary

Fixes #316.

Opening this after claiming the issue two days ago; the reporter confirmed the exact reproduction in the meantime (/skill:foo @ on the same line shows no completions). Happy to adjust or close if a different fix is planned.

Root cause

Not stale state - a routing gap in build_completion_state (src/tau_coding/tui/autocomplete.py). Any text starting with / never reaches the @ file completer, and a matched /skill: command with argument text returned an empty completion state. So @ references, which work in normal prompt text, offered nothing inside a skill invocation.

Fix

Skill arguments are prompt text, so that branch now falls through to the file-reference completer (the TUI already passes cwd, so it works end to end):

if token.startswith("/skill:"):
    if has_argument_text and _matches_skill_command(token, skills):
        # Skill arguments are prompt text, so @ file references stay available.
        if cwd is not None:
            return CompletionState(_file_reference_completions(text=text, cwd=cwd))
        return CompletionState()

/skill:review fix @app now suggests @src/app.py. Generic slash commands like /help @read keep completions off - that behavior is deliberate and its existing test (test_file_reference_completion_stays_off_for_slash_commands) is untouched.

Scope note

The reporter also raised skill chaining (/skill:foo /skill:bar in one line). That is new command-parsing semantics rather than a completion-routing bug, so it is deliberately out of scope here (noted on the issue).

Tests

• New regression test test_file_reference_completion_works_in_skill_command_arguments, written first and confirmed failing before the fix.
• Full tests/test_tui_autocomplete.py: 30 passed. ruff and mypy clean. Rebased on current main.

🤖 Generated with Claude Code