Add Agent API workflow cookbooks

[gleb-reshetnev]

What

Adds three runnable Agent API workflow tutorials with application-owned state.

Was: The cookbook did not show how to adapt the OpenAI Data Analyst, Slack Bot, and SRE Agent workflows to the Perplexity Agent API.
Now: Users can run tested Python examples for read-only analysis, channel-scoped Slack answers, and persisted operator decisions.

• Executes custom functions through a bounded background-response loop with validated arguments, transcript checkpoints, cancellation, and retry handling.
• Enforces read-only SQLite queries, explicit saved corrections, and persisted follow-up conversations.
• Binds Slack search to the verified workspace and triggering channel, deduplicates events, and supports queued stop requests.
• Persists exact incident proposals and approve or deny decisions with no deployment execution by design.
• Includes nine offline workflow tests and a paid live smoke check using synthetic data and simulated Slack transport.

Why

Changing only the SDK endpoint does not reproduce OpenAI Agents API session behavior. These examples provide the missing application-owned conversation, tool-result, cancellation, and approval state without claiming unsupported session or executor parity.

Real Slack delivery remains an explicit manual integration check because this change has no test-workspace credentials.