fix(prompt): simplify pickup follow-through (#6186)

## Why

The base prompt prescribed a todo lifecycle for every task needing
follow-up tools, which added control calls even for work completed in
one turn. This keeps the important behavioral contract—continue after
publishing pickup—without prescribing the mechanism.

## What

- Replace the 40-word todo lifecycle with a concise pickup
follow-through rule
- Preserve the requirement to publish the outcome or blocker before
stopping

## Risk Assessment

Low to medium — this changes managed-agent instructions, not runtime
code. The terminal benchmark covers single-turn task completion but does
not cover restart, compaction, or long-lived multi-turn recovery.

## References

- Builds on the prompt simplification in
https://github.com/block/buzz/pull/6161
- Benchmark setup: GPT 5.6 Terra at high effort; the same 11-task
Terminal-Bench 2.1 slate; four concurrent trials; 4 CPU and 8 GiB per
trial; 3× timeout

| Prompt | Pass | Active-h | Median active | Tool calls |
|---|---:|---:|---:|---:|
| PR 6161 baseline | 22/22 | 0.3438 | 0.91 min | 216 |
| Benchmarked 14-word rule | 22/22 | 0.2926 | 0.75 min | 198 |

The benchmarked rule used 14.9% less active time, 16.9% lower median
active time, and 8.3% fewer tool calls. Across the screen and
confirmation runs it passed 33/33 trials with every completion report
present; results are directional because model and tool behavior is
stochastic.

---
**Update Aug 18, 10:47 EDT:** Expanded the completion outcomes following
review feedback.
- The follow-through rule now covers a verified result, blocker, or key
decision or information that needs to be surfaced.
- The benchmark was not rerun; the table reflects the prior 14-word
formulation. This is a completion-taxonomy clarification, not a return
to a prescribed todo mechanism.

Generated with Codex

Signed-off-by: Salman Mohammed <smohammed@squareup.com>

crates/buzz-acp/src/base_prompt.md

@@ -80,7 +80,7 @@ All replies and delegations — including task assignments to other agents — g
 - **Otherwise, publishing is optional and silence is usually correct.** When a message leaves you nothing new to contribute, end the turn without publishing. That is a success, not a failure.
 - **After a context compaction or session restart, resume silently** — rebuild state from your todos, memory, and the thread, and never post a message announcing the compaction, summarizing what was lost, or asking how to proceed.
 - **Never publish a bare acknowledgement.** A message whose only content is confirming, accepting, agreeing, aligning, signing off, or announcing your own silence adds nothing — and it re-triggers everyone you mention. Prohibited: "Got it", "Confirmed", "Acknowledged", "Clear and noted", "Aligned", "Standing by", "Parked", "I won't reply again", and any variation. If your draft contains nothing beyond acknowledgement, send nothing. If you are tempted to announce that you are done replying, that itself is the message not to send.
-- For work that requires follow-up tools, create an open todo **before** sending the pickup acknowledgment. Keep it open until the deliverable is verified and you have sent a completion or blocker message; never end a turn with open todo state unless you have posted that completion or blocker message.
+- After publishing a pickup message, keep working until you publish the verified result, blocker, or key decision or information that needs to be surfaced.
 - Use GitHub-flavored Markdown. Fenced code blocks with language tags for syntax highlighting.
 - No push notifications — poll with `buzz messages get --channel <UUID> --since <ts>`.
 - Address people using the name shown in their own message header. Preserve it exactly; do not infer, expand, or look up a surname merely to address them.