feat(chat): fold checkpoint resume into the input — Continue + abandon (#255)

[cboettig]

Fixes #255.

Problem

When the agent pauses at a checkpoint, the UI rendered a standalone ▶ Continue button inside the message stream — competing with the chat input even though sending any message already resumes the suspended turn (same code path). And once paused, there was no way to abandon the suspended work: any new message got folded into the old turn as a steer, with no escape short of reloading.

Change

The resume affordance now lives in the input row:

• 3-state send button via a single _syncInputControls():
  • idle → Send
  • busy → ■ Stop (abort, unchanged)
  • suspended + idle → Continue (green)
• Continue / Enter on an empty input resumes the paused turn ("continue"); typing a steer first resumes with that steer instead.
• New ✕ abandon control, visible only while suspended — clears agent.suspendedTurn so the next message starts a fresh turn.
• Placeholder hints the affordance while paused.
• Removed the standalone renderContinueButton / .checkpoint-actions block and its now-dead CSS.

State that was scattered through handleSend (the busy on/off button mutations) is now centralized in _syncInputControls().

Tests

test/chat-ui-controls.test.js (jsdom) drives the prototype methods on a minimal instance (mirroring the map-manager unit tests):

• _syncInputControls: idle→Send, busy→Stop (abandon hidden even if suspended), suspended→Continue+abandon+hint.
• abandonSuspendedTurn: clears the turn / resets to Send / notes it; no-op while busy.
• handleSend: empty input resumes with "continue"; empty + no suspended turn is a no-op; a typed steer wins over the canned resume.

Full suite green (334).

Manual verification (browser-bound, per AGENTS.md)

On a downstream app with a low max_tool_calls (e.g. 3) so a checkpoint triggers:

1. Ask a multi-step question → agent pauses → send button reads Continue, ✕ appears, placeholder hints resume.
2. Press Continue (empty input) → resumes.
3. Repeat; this time type a steer first → resumes with the steer.
4. At a pause, press ✕ → system note, button returns to Send, next message starts a fresh turn.
5. While a turn is running, button is ■ Stop and ✕ is hidden.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	51.39%	3749 / 7295
🔵	Statements	51.37%	3749 / 7298
🔵	Functions	59%	131 / 222
🔵	Branches	82.67%	754 / 912

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
app/chat-ui.js	32.76%	65%	25%	32.82%	109-133, 140-224, 227-239, 256-328, 335-365, 372-417, 424-433, 436-442, 445-502, 508-530, 537-555, 562-590, 596-598, 613-615, 624, 635-636, 641-642, 648-657, 714-738, 746-772, 779-797, 800-802, 805-806, 809-810, 817-819, 826-845, 858-937, 952-958, 963-964, 1001-1002, 1010, 1027-1072, 1079-1106, 1109-1114, 1117-1122, 1131-1184, 1191-1196, 1199-1210, 1217-1220, 1228-1236, 1243-1278, 1288, 265-266

Generated in workflow #111 for commit db97ef7 by the Vitest Coverage Report Action