examples(inference): add playground inference agent

[rosetta-livekit-bot]

Summary

Turns the Inference example into a lightweight agent-builder playground that pairs with the corresponding Jukebox PR (livekit/agents-jukebox#TODO).

• Editable system prompt. Adds a set_system_prompt RPC that rewrites agent.instructions live. Deduped against the current value so the per-keystroke fires from the frontend don't spam update_instructions.
• "Open in Builder" CTA. New type: link control in the YAML with an href_template that opens Cloud Builder pre-populated with the current instructions (URL-encoded), llm, stt, tts, and modelMode=pipeline. project_id is the p_ placeholder so Cloud routes through login when needed and preserves the params on redirect.
• Live transcript view. Adds a views[].source: transcript entry; the Jukebox frontend renders LiveKit TranscriptionReceived segments into the existing view-card slot — no agent-side push needed.
• Defaults bumped. cartesia/sonic-2 → cartesia/sonic-3, openai/gpt-4o-mini → openai/gpt-4.1-mini, both in playground.yaml and the agent's DEFAULT_* constants. Seed INSTRUCTIONS aligned with the textarea's default so the first session matches what the user sees before any edit.

YAML schema also picks up doc comments for the new textarea / link control types and the transcript view source — the Jukebox PR has the corresponding parser + renderer changes.

Test plan

• cd examples/inference && python agent.py dev — connect via Jukebox playground; verify prompt edits land (system prompt updated (N chars) in agent logs) and the agent's behaviour changes within a turn.
• Pick a new STT / LLM / TTS — confirm the model swap still works after the prompt-editing changes.
• Click "Open in Builder" — confirm a Cloud Builder URL opens in a new tab with instructions, llm, stt, tts, and modelMode=pipeline matching the current Playground state.
• Default-only session (no edits) — verify the agent's initial prompt matches the textarea's seed.

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[devin-ai-integration]

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