feat: input cursor navigation, paste handling, word-boundary movement

- Left/Right arrows move cursor by char boundary, Backspace deletes at
  cursor, Char inserts at cursor (not end-of-string)
- Home/End jump to start/end, Ctrl+Left/Right skip by word
- Cursor renders at the correct visual position (wrapping-aware)
- Bracketed paste detection (crossterm 0.27 feature) — pastes >3 lines
  tagged with a system message so the agent sees pasted material
- Paste inserts at cursor position with no size cap
- docs/bugs.md updated: B-006 fixed
- CLAUDE.md, docs/tasks/INDEX.md updated for refactor status

Cargo.toml

@@ -56,7 +56,7 @@ hound = "3.5"
 # tokenizers = "0.15"  # For local tokenization
 
 # Terminal/UI
-crossterm = "0.27"  # Terminal control
+crossterm = { version = "0.27", features = ["bracketed-paste"] }  # Terminal control (bracketed paste for paste detection)
 ratatui = { version = "0.30", features = ["crossterm"] }  # TUI framework with crossterm backend
 unicode-width = "0.1"
 arboard = { version = "3", features = ["wayland-data-control"] }  # Clipboard — click-to-copy a message bubble