--file and --dir are not mutually exclusive, so --dir is silently ignored

[royalpinto007]

Desired outcome

Passing both --file and --dir is rejected instead of silently ignoring one of them.

Why it matters

_load() in agentrace/cli.py:

if args.file:
    sessions = [parse_session(Path(args.file))]
else:
    sessions = parse_all(Path(args.dir) if args.dir else None)

--file wins and --dir is discarded without a word. Both are defined as plain top-level options in main(), so nothing stops a user from writing:

agentrace check --dir ./transcripts --file ./transcripts/session.jsonl

and this is a natural thing to type when narrowing down from a directory run to a single file: you add --file and leave the --dir you already had in your shell history. The output then describes one file while the user believes it covers the directory. For a tool whose whole purpose is telling you which agent results not to trust, quietly analysing a different input than the one requested is a bad failure mode.

The empty-result message compounds it: _load prints "Looked in ~/.claude/projects unless --dir was given" even on the --file path, so a typo'd filename reports the wrong location.

Steps

1. In main(), put --file and --dir in a mutually exclusive group:

   src = p.add_mutually_exclusive_group()
   src.add_argument("--dir", ...)
   src.add_argument("--file", ...)

   argparse then errors out with a clear message and no code in _load needs to change.

2. Make the "no runs found" message name the actual source: the file path when --file was used, the directory otherwise.

3. Consider erroring when --file points at something that does not exist, rather than letting it surface from parse_session.

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