docs(cli): complete CLI exit codes documentation in docs/cli.md (#107)

[PrinceThummar011]

Summary

Completes the CLI exit codes documentation in docs/cli.md per issue #107, ensuring
scripts and CI/CD pipelines have accurate, complete reference for command exit statuses.

Problem

While docs/cli.md already contained a partial exit codes table (0–5), it was missing:

• Exit code 6 (CorpusError), which is actively used in the codebase
• A short introductory line explaining the purpose of the table

Changes

• Added exit code 6 (Corpus error) to the exit codes table, sourced directly from
  the _ERROR_CODES mapping in openagent_eval/cli/main.py
• Added a contextual intro line above the table: "The CLI returns the following exit
  codes so scripts and CI/CD pipelines can check command results."

Verification

• ✅ Cross-referenced documented codes against _ERROR_CODES in openagent_eval/cli/main.py
• ✅ Manually confirmed exit codes at runtime:
  • --version → 0
  • invalid command → 2
  • invalid config path → 2
• ✅ uv run mkdocs build — docs build passes cleanly
• ✅ uv run pytest tests/unit/test_cli/ — 107/107 tests passing

Notes

A prior claim on this issue reported the exit codes table already existed. This PR
verifies that claim against the source code and closes the remaining gap (missing
code 6 and context line) rather than duplicating existing work.

Closes #107

[Copilot]

Pull request overview

This PR updates the CLI reference documentation to fully describe the exit codes that oaeval can return, aligning the docs with the CLI’s actual error-to-exit-code mapping so scripts and CI/CD pipelines can reliably interpret outcomes.

Changes:

• Adds a short introductory sentence explaining the purpose of the exit code table.
• Updates the exit code table to include code 6 for “Corpus error”.
• Refines the description for exit code 1 to “General or runtime error”.

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[github-actions]

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