fix(web): stop rendering sidechain prompt as user message

[hqhq1025]

Summary

• Sub-agent (sidechain) prompts were incorrectly converted to user-text blocks in reducerTimeline.ts, causing them to appear as standalone messages in the web chat UI
• The prompt text is already visible in the parent Task tool call's input, so this was a duplicate/misattributed display
• Fixed by skipping sidechain content blocks instead of creating user-text blocks

Test plan

• Open a session that uses sub-agents (Agent tool) in the web UI
• Verify the sub-agent prompt no longer appears as a separate user message
• Verify the Task tool call still shows the prompt in its input parameters
• Verify sub-agent responses still render correctly as children of the Task tool call

[github-actions]

Findings

• None.

Summary
Review mode: initial. No issues found in diff. Residual risk: UI behavior change untested; manual verification recommended.

Testing

• Not run (automation)