Fix worker completion results not reaching users

[jamiepine]

Summary

• Fix retrigger prompt that let the LLM skip or send empty teasers instead of relaying worker results to the user
• Add code-level fallback: if LLM skips on a retrigger turn but produced text, send it anyway
• Change channel prompt to always acknowledge worker spawns (brief reply like "On it") instead of silently skipping

What was happening

Two failure modes observed on worker completion retriggers:

1. LLM calls skip thinking the [Worker completed] message was "already processed" — user gets nothing
2. LLM calls reply with a 75-char teaser ("Here's the issues:") referencing the result as if the user can see it — user gets a stub

Root cause: the retrigger prompt said "skip if the result doesn't add meaningful new information" and the channel prompt said "prefer skip and wait". The LLM treated [Worker completed] history entries as user-visible.

Changes

Prompt fixes:

• retrigger.md.j2 — explicitly states user has NOT seen the result, must relay full substance via reply
• channel.md.j2 line 39 — tells LLM the [Worker completed] message is invisible to users
• channel.md.j2 rule 4 — always reply with brief ack after spawning a worker, only skip for branches

Code safety net (channel.rs handle_agent_result):

• New skipped && is_retrigger branch: if LLM skips on retrigger but produced text output, sends it as fallback
• Warns when LLM skips on retrigger with no text at all

Also includes workers tab design doc and minor UI fixes for worker display overflow.

Note

Automated Summary

This PR addresses a critical issue where worker completion results were not reaching users due to two LLM behavior problems on retrigger turns. The fix involves three layers: clarifying prompts to explicitly state that worker results must be relayed to users, updating channel communication rules to distinguish between worker acknowledgments (always reply) and branches (skip), and adding a code-level safety net that sends worker results as fallback text if the LLM incorrectly skips on retrigger turns. Additional changes include UI overflow fixes for worker displays and a comprehensive design doc for the workers tab feature that will track worker runs with event timelines and full transcripts.

_{Written by Tembo for commit 3e80351. This will update automatically on new commits.}