feat(tui): next-3 round — attention layer, linear mode, diff fences

[jkyberneees]

Summary

The three improvements ranked by the expert panel (NEXT_3_FEATURES.md), each implemented TDD-first with the README synced in the same commit.

1. Attention layer

• Turn completion and pending approvals set the terminal window title (✓ done — <model> / ⚠ approval needed — <model>) and ring the bell; --bel=false mutes the bell, --notify adds OSC 9 desktop notifications.
• One raw tea.Exec write per event — terminal states only, never per token. Wire-borne strings pass through sanitize/collapse so control bytes cannot inject terminal escapes.

2. --plain linear mode

• No alt-screen: agent events print append-only to the terminal's native scrollback above a minimal input chrome. Text-prefix severity ([error], ⚠ approval, ▸ tools, ❯ prompts, ✓ done · N tools · Xs · N tok).
• Streamed fragments suppressed; the reply lands whole when the turn ends. The accessible surface for screen readers — and the natural one for pipes (bodek --plain < task > run.log).
• NO_COLOR now deterministically degrades the whole palette in every mode.

3. Diff-aware rendering

• Fenced ```diff blocks tint inside prose without mis-styling the surrounding text (fence-first dispatch), fences without @@ hunk headers render as diffs (fence is authoritative), and the +N −M step chip counts fenced stats.

Test plan

• make fmt clean · make vet clean · make lint 0 issues
• make test — go test -race ./..., all packages green
• New suites: attention_test.go (10 tests), plain_test.go (9), renderers_diff_test.go (6), cmd flag tests (3)