feat(acp): add v2 mid-turn steering

[danielkov]

Summary

• adds ACP v2 new, list, resume, prompt, cancel, close, and configuration handling
• adds session/inject and mandatory session/revoke_inject with steer and finish behavior
• delivers accepted steers FIFO after the active stream or tool call and before the next model request
• moves the bundled TUI to ACP v2 and lets Enter submit steers during active work
• shows accepted steers above the input until their user_message delivery moves them into the transcript
• namespaces persistent sessions by workspace and migrates older transcript formats

Compatibility

• kit acp and kit serve still default to ACP v1 for clients such as Zed
• the HTTP ACP endpoint negotiates v1 or v2 from initialize
• only the bundled TUI explicitly starts a v2 stdio server

Dependencies

• RFD: docs(rfd): mid-turn input via session/inject (queue and steer) agentclientprotocol/agent-client-protocol#1261
• Schema: feat(unstable-v2): add session injection schema agentclientprotocol/agent-client-protocol#2043
• Rust SDK response receipts: feat(jsonrpc): add tracked response receipts agentclientprotocol/rust-sdk#338
• Rust SDK injection support: feat(unstable-v2): add session injection support agentclientprotocol/rust-sdk#339
• AgentKit v2 foundation: feat(acp): add opt-in ACP v2 runtime foundation danielkov/agentkit#12
• AgentKit injection support: feat(acp): add unstable v2 session injection danielkov/agentkit#13

The current build pins AgentKit 4299f31ec1455ea8ee4c46b4dce8e5bec4fdc597, Rust SDK 2f039993d1d6ed8da35b38c31f54a7cbb7338c70, and schema 6e7e044f9464c4fd652d90699a09e9edc8b3bbad.