Reject ambiguous transcript source options

[vsolano9]

What & why

Passing both --file and --dir silently analyzes only the file, while empty and missing-file paths report misleading sources. This makes the input selection unambiguous at argparse’s boundary and keeps errors honest.

Closes #8

Changes

• put --file and --dir in one mutually exclusive argparse group
• reject a missing --file with a clean exit-2 parser error and no traceback
• name the selected file, directory, or default root when no runs are found
• escape source paths before Rich markup and prevent soft wrapping from splitting them
• add four real CLI subprocess regressions and update the documented test count

Validation

• RED: all four new CLI cases failed against the original implementation
• pytest -q — 22/22 pass; repeated successfully 20 consecutive times
• official fixture list and stats --json smoke commands — pass
• focused Ruff lint for both changed code files — pass
• Ruff format for the new test file — pass
• wheel build and pip check — pass
• git diff --check — pass
• current full-repository Ruff check/format remains red only on pre-existing untouched style drift under Ruff 0.16.4; the repository CI does not run Ruff

Checklist

• Focused change with a clear description
• Ran the repository test and CLI smoke steps
• Added regression tests for changed behavior
• Updated documentation where relevant
• Linked the related issue

[github-actions]

Welcome, @vsolano9, and thanks for your first pull request to Agentrace.

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