feat: enrich human gates with Markdown rendering and file links

[PolyphonyRequiem]

Summary

Enriches human gate prompts with proper Markdown rendering and interactive file links in both the terminal CLI and web dashboard.

Changes

Terminal (CLI):

• Gate prompts now render as Rich Markdown — headings, bold, lists, code blocks, and links display properly instead of raw markup text

Web Dashboard:

• New \GET /api/files/{path}\ endpoint serves local files relative to the workflow root
  • Security: path traversal protection (
    esolve()\ +
    elative_to()), extension allowlist, 1MB size limit, absolute/UNC/scheme path rejection
• New \FileViewer\ modal component renders linked files (Markdown for .md, monospace for others)
• Relative links in gate prompts (e.g. [plan](./plan.md)) open the FileViewer instead of navigating away
• External URLs continue to open in new browser tabs

Files Changed

File	Change
\src/conductor/gates/human.py\	Wrap prompt in \RichMarkdown()\
\src/conductor/web/server.py\	Add \workflow_root\ param + file API endpoint
\src/conductor/cli/run.py\	Pass \workflow_root\ to WebDashboard
\src/conductor/web/frontend/.../FileViewer.tsx\	NEW modal file viewer
\src/conductor/web/frontend/.../GateDetail.tsx\	Link interception + \isRelativeFileLink\ helper
\ ests/test_web/test_server.py\	13 new file API security tests
\ ests/test_gates/test_human.py\	3 new/updated Markdown rendering tests

Testing

All 79 tests pass across both test suites (test_server + test_human). No regressions.