refactor(tui): replace content_buffer String with structured ChatMessage model

Core architecture change — the TUI now uses a typed message list
instead of a flat String buffer for conversation display:

New data model:
  ChatMessage::User { text }         → rendered as "❯ {text}"
  ChatMessage::Assistant { text }    → rendered as "● " + markdown
  ChatMessage::ToolUse { name }      → rendered as "● {name}" dim
  ChatMessage::ToolResult { ... }    → collapsible output (10 line max)
  ChatMessage::System { text }       → dim italic

Benefits:
  - Each message type has independent rendering logic
  - Streaming ContentDelta appends to last Assistant message
  - No more string prefix matching for style classification
  - ● marker added exactly once per assistant response
  - Tool results properly typed with is_error flag
  - Foundation for message-level folding, selection, rewind

Event handlers now push to messages Vec instead of appending
to content_buffer. render_messages() replaces render_content_scrolled().
Old content_buffer kept as dead_code for gradual removal.

Author: lingcoder