feat(acp): terminal streaming lifecycle events in tool call _meta

[bug-ops]

Part of epic: #1007

Problem

AcpShellExecutor blocks until the command completes, then returns the full output. IDEs receive terminal output only after execution finishes — no real-time streaming visible in the tool card.

Solution

Introduce a per-call streaming output sink. For each shell tool call, emit:

1. ToolCall with _meta.terminal_info = { terminal_id } on start
2. ToolCallUpdate with _meta.terminal_output = { terminal_id, data } per stdout/stderr chunk
3. ToolCallUpdate with _meta.terminal_exit = { terminal_id, exit_code, signal } on finish

Protocol shape (from Zed claude-agent-acp):

// ToolCall
{ "_meta": { "terminal_info": { "terminal_id": "<uuid>" } } }

// ToolCallUpdate (streaming)
{ "_meta": { "terminal_output": { "terminal_id": "<uuid>", "data": "chunk" } } }

// ToolCallUpdate (final)
{ "_meta": { "terminal_exit": { "terminal_id": "<uuid>", "exit_code": 0, "signal": null } } }

Tasks

• Generate terminal_id (UUID) in agent.rs before shell tool dispatch
• Refactor AcpShellExecutor::execute() to accept output_sink: impl Fn(String) + Send callback in crates/zeph-acp/src/terminal.rs
• Sink emits ToolCallUpdate { _meta: terminal_output { data } } via ACP client per chunk
• On command exit: emit ToolCallUpdate { _meta: terminal_exit } with exit code
• Integration test: verify terminal_output chunks arrive before final ToolCallUpdate

Files

• crates/zeph-acp/src/terminal.rs — AcpShellExecutor streaming refactor
• crates/zeph-acp/src/agent.rs — terminal_id generation + _meta serialization in prompt()