feat: update app manifest to use agent_view

[mwbrooks]

Summary

This pull request flips all three Casey implementations from assistant_view to agent_view to opt into the new Agent DM Messages Tab experience.

• Rename features.assistant_view → features.agent_view in every manifest, and the inner assistant_description → agent_description.
• Remove assistant_thread_started from settings.event_subscriptions.bot_events.
• Merge the existing set_suggested_prompts logic into the app_home_opened handler, gated on event["tab"]: the Home tab keeps publishing its Block Kit view (category buttons + MCP status), the Messages tab pins the suggested prompts.
• Delete the now-unused assistant_thread_started.py handler files.
• Refresh tests/test_app_home_opened.py to cover both event["tab"] branches.
• TODO(agent-dm-messages-tab) markers placed where app_context_changed wiring will land once Bolt for Python exposes the event type. That work is deferred to a follow-up PR.
• README screenshots will be refreshed after the partner flip lands and we can capture the new UX.

Implementation note: Bolt's set_suggested_prompts middleware helper is only injected for assistant_thread_started, so the handler now calls client.assistant_threads_setSuggestedPrompts on the WebClient / AsyncWebClient directly.

Files touched per implementation:

claude-agent-sdk/ (async)

• manifest.json — agent_view rename, drop assistant_thread_started subscription
• listeners/events/app_home_opened.py — branch on event["tab"]
• listeners/events/__init__.py — drop assistant-thread registration
• listeners/events/assistant_thread_started.py — deleted
• tests/test_app_home_opened.py — cover both branches

openai-agents-sdk/ (sync) — same change set
pydantic-ai/ (sync) — same change set

Testing

Manual verification before review:

• cd claude-agent-sdk && pip install -r requirements.txt && ruff check . && ruff format --check . && pytest
• cd openai-agents-sdk && pip install -r requirements.txt && ruff check . && ruff format --check . && pytest
• cd pydantic-ai && pip install -r requirements.txt && ruff check . && ruff format --check . && pytest
• Boot smoke: python app.py from each subfolder fails only on missing tokens/provider keys (event listeners register without throwing).
• End-to-end validation against a real workspace will be done in a sandbox after merge (deferred — the partner flip lands ~2026-06-23).