#19535 Agent pane: past-chat reader on AgentPanePastTurn, fixes package-conventions-lint base red (cx-5esm)

Summary

This fixes a base red that #19518 introduced. package-conventions-lint flagged AgentPanePastChat as a namespace type: an all-static enum that is never instantiated. The ios-tests job fails on the same lint.

The reader closure and turns(of:) move onto AgentPanePastTurn, the type they read, and the enum is gone. The three call sites change, and behavior stays the same.

Testing

• scripts/lint-ios-package-conventions.sh fails on base and passes here ("OK: no unjustified convention violations").
• The Release compile runs as the fresh-merge check on the merged head.
• The Swift suites run on the fleet (AgentPanePastChatTests uses the renamed reader).

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Fixes a base red that #19518 introduced: package-conventions-lint flagged AgentPanePastChat as a namespace enum that is never instantiated, failing the ios-tests job.

• Moves the past-chat reader and turns(of:) onto AgentPanePastTurn, the type they read; the enum is removed.
• Updates the three call sites; behavior is unchanged.