🤖 fix: prevent retry barrier flashing

[ammar-agent]

Summary

• extend the pending stream-start grace period and document it via a shared constant
• expose markPendingStreamStartFailed through WorkspaceStore and call it from ChatInput when send fails
• clarify that plan mode responses must return the completed plan in AGENTS.md

Generated with mux

Testing

• make typecheck
• bun test src/browser/utils/messages/retryEligibility.test.ts

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[ammar-agent]

Addressed the Codex warning by keeping pendingStreamStartTime intact until a stream actually starts (or the user retries). That removes the retry-loop regression and still hides the RetryBarrier during the extended 15s grace window.