Fix ACP agent tab chat binding

[Finesssee]

Summary

• Rebinds temporary ACP agent tabs to the real chat id once a message creates or selects a chat.
• Rebinds the active agent tab when selecting a new ACP provider from the header selector.
• Keeps the visible chat pane, tab title, and provider-specific controls aligned with the chat that receives the prompt.
• Adds provider identity to assistant/loading/tool rows so ACP responses do not render as bare floating text.

Validation

• bun typecheck
• git diff --check
• bunx tauri build --debug --config src-tauri/tauri.dev.conf.json (build completed; bundle signing failed locally because the Developer ID identity is not installed, then the debug app was ad-hoc signed for local UI verification)

UI smoke

Verified locally with the debug Athas app opened on /Users/sw/Code/athas via Computer Use:

• Selecting OpenCode updates the visible chat controls to OpenCode and sends the prompt to the OpenCode chat. OpenCode currently returns a visible EMPTY_RESPONSE provider error.
• Selecting GitHub Copilot creates a GitHub Copilot chat, sends hi, and renders the visible assistant Hi response in the chat pane.
• Selecting Codex CLI sends hi and renders the loading/assistant response with a visible provider icon row instead of bare text.

[Finesssee]

Superseded by #682. I merged the useful chat-message polish into the ACP runtime/catalog branch while keeping #682's selector routing, so this branch no longer needs to land separately.

[Finesssee]

Closing as superseded by #682, which now includes the retained chat polish without the selector conflict.