Add mountaineer lint CLI command with actionable feedback

[pradhankukiran]

Summary

• add a dedicated mountaineer click entrypoint with a new lint command
• implement handle_lint pipeline support for ruff, pyright, and mypy with timing + pass/fail output
• add actionable guidance parsing for common diagnostics and dedupe noisy repeated lines (notably pyright)
• support --path (multi), --fix, and --fast flags
• default lint target selection now avoids hidden dirs and focuses on package directories to prevent accidental .venv linting
• document the new CLI command in API docs

Tests

• uv run pytest mountaineer/__tests__/test_lint.py mountaineer/__tests__/test_entrypoint.py
• uv run pytest mountaineer/__tests__
• cd create_mountaineer_app && uv run pytest
• manual CLI checks:
  • uv run mountaineer --help
  • uv run mountaineer lint --help
  • uv run mountaineer lint --fast --path mountaineer/__tests__/test_entrypoint.py
  • uv run mountaineer lint --path mountaineer/__tests__/test_lint.py
  • uv run mountaineer lint --fast
  • uv run mountaineer lint --fix --fast --path /tmp/... (verified both failing non-fixable and passing fixable cases)

Closes #219