Preview referenced files in a modal (e.g. click plan.md to view it)

[SGudbrandsson]

Problem

When Claude (or any transcript content) mentions a file like plan.md, there's no way to view it from within Codeman. I want to click the filename and see the contents inline.

Proposal

• Detect file path references in transcript output and render them as clickable links.
• Clicking opens a modal preview with the file contents (syntax-highlighted / markdown-rendered where appropriate).
• Preview should work for files inside the session's worktree / project directory.

Acceptance criteria

• File references in chat are clickable.
• Clicking opens a modal showing the file contents.
• Markdown files render as markdown; code files render with syntax highlighting.
• Modal closes cleanly (Esc, backdrop, close button).
• Safe handling for files outside the session's allowed scope (no arbitrary filesystem reads).