feat(cli): support Agent mode switching in Shared TUI

[limityan]

Summary

• Let Shared TUI change the current Session Agent mode through /agent, Tab, and Shift+Tab.
• Reuse the existing Agent selector and Agent Runtime owner; the private CLI IPC protocol adds only the reviewed Session mode update operation.
• Require the controlling, idle Session before applying a change, and update visible TUI state only after the Runtime confirms success.
• Treat timeout or disconnect as an unknown outcome: close the Shared connection and tell the user to restore the Session and inspect its mode before retrying.
• Keep pending mode updates from sending a turn, opening Sessions, running /init, or switching Sessions through the same IPC request path.

Scope

This does not add model selection, Agent/Subagent management, GUI, Headless/CI, ACP, SDK Host, Peer/Remote, or public SDK/protocol support. Embedded TUI behavior remains unchanged.

Validation

• cargo check -p bitfun-agent-runtime-ipc -p bitfun-cli
• cargo test -p bitfun-agent-runtime-ipc --lib — 29 passed
• cargo test -p bitfun-cli actions::tests — 31 passed
• cargo test -p bitfun-cli modes::chat::tests — 58 passed
• cargo test -p bitfun-cli --test product_assembly_cli — 10 passed
• Core boundary check and its 16 self-tests
• Repository hygiene and git diff --check
• Independent architecture and product reviews: no remaining P0/P1/P2 findings

Repository-wide verification remains limited by existing unrelated Windows CLI dispatch/store failures and the external sherpa-onnx-sys workspace build wait; the affected CLI/IPC paths above are green.