feat(acp): release ACP v2 session injection behind unstable-inject

[danielkov]

Lands the ACP v2 session injection stack (previously #13, kept unmerged behind a git patch) and David's inline media budget (#25), now depending on published fork crates instead of git revisions.

• agentkit-acp depends on agentkit-acp-sdk 2.2.0: the upstream agent-client-protocol SDK (current main) plus session injection, tracked response receipts and bounded transports. It uses agentkit-acp-schema 1.9.1 for the schema. Library names are unchanged, so code still imports agent_client_protocol.
• Injection stays behind the unstable-inject feature; default features are unchanged apart from the SDK moving from 2.0 to 2.2.
• Upstream ACP v2 now requires session/prompt responses to carry the accepted user message id. The id is allocated when the prompt is accepted and echoed by the later user_message update.
• Loop: preserve and bound interrupted streamed responses, and retire interrupted turns without resuming execution.
• Bumps agentkit-acp to 0.10.12 and agentkit-loop to 0.10.13.