feat(tui): Sprint 2 — markdown rendering, ConversationContent enum, diff viewer
- Create src/markdown.rs with MarkdownRenderer using pulldown-cmark + syntect
(lazy-loaded via once_cell::Lazy, theme fallback chain, syntect 5.x API)
- ConversationContent enum: Plain/Markdown/CodeDiff variants
- push_output() auto-detects markdown via looks_like_markdown() heuristic
- build_wrapped_conversation() routes through MarkdownRenderer for markdown entries
- render_diff() for syntax-colored unified diffs (green+/red-/cyan @@)
- push_diff() convenience method for /diff command
- once_cell added to Cargo.toml
- 27 new tests for markdown rendering, detection, and diff colors
- All existing tests pass (240 unit tests, 108 contract tests)
Sprint 2 of 7.
Authored by TheArchitectit
e8e8c72185