Render Markdown and add task manager preview

[SSHdotCodes]

What changed

• render common Markdown in the lightweight native task manager
• add safe clickable web links and reject unsafe URI schemes
• add focused parser tests for headings, emphasis, lists, tables, code, ANSI output, and partial Markdown
• add a real task-manager screenshot to the README and document the rendering behavior

Why

The task transcript previously displayed Markdown markers literally, and the README screenshot existed only locally, so GitHub could not render it.

Impact

Agent output is easier to scan without adding a browser or third-party Markdown dependency. The README now includes the actual app view, with the image committed in the same branch.

Validation

• python3 -m py_compile bin/second-opinion
• python3 -m unittest discover -s tests -q (31 tests)
• git diff --check
• visual macOS task-manager verification
• 120 KB Markdown transcript parse benchmark (~23 ms, ~2.15 MiB peak under tracing)