feat(acp): subagent visibility in IDE via ACP protocol

[bug-ops]

Overview

Zeph has a mature ACP implementation but subagent activity is invisible to IDEs. When the orchestrator spawns subagents (skill delegation, multi-step tasks), the IDE sees flat opaque tool calls with no hierarchy, no real-time terminal streaming, and no file navigation.

This epic tracks bringing Zeph ACP to parity with the Zed claude-agent-acp reference implementation.

Background

Reference: https://github.com/zed-industries/claude-agent-acp

The three missing capabilities:

1. parent_tool_use_id propagation — IDE nests subagent output under parent tool call card
2. Terminal streaming lifecycle (terminal_info / terminal_output / terminal_exit in _meta) — real-time bash output in IDE tool cards
3. ToolCallLocation (filePath + line range) — IDE editor cursor follows agent across files

Acceptance Criteria

• IDE chat view shows subagent output nested under parent tool call card
• IDE terminal view streams bash output in real time (no buffer-then-show)
• IDE editor cursor follows agent when read_file/write_file tool fires
• parent_tool_use_id is null for top-level agent, <uuid> for subagents
• All existing 40+ ACP unit tests pass unchanged
• cargo clippy --workspace -- -D warnings passes
• cargo nextest run --workspace --lib --bins passes

Files Affected

File	Change
crates/zeph-acp/src/agent.rs	LoopbackEvent variants, prompt() handler, _meta serialization
crates/zeph-acp/src/terminal.rs	Add streaming output sink to execute()
crates/zeph-acp/src/fs.rs	Return file path for ToolCallLocation
crates/zeph-core/src/agent/	Inject parent_tool_use_id when spawning subagents

Child Issues

• feat(acp): propagate parent_tool_use_id through LoopbackEvent and SessionUpdate #1008 feat(acp): propagate parent_tool_use_id through LoopbackEvent and SessionUpdate
• feat(acp): terminal streaming lifecycle events in tool call _meta #1009 feat(acp): terminal streaming lifecycle events in tool call _meta
• feat(acp): add ToolCallLocation to file operation tool calls #1010 feat(acp): add ToolCallLocation to file operation tool calls
• docs(acp): IDE integration guide and validation for subagent visibility #1011 docs(acp): IDE integration guide and validation for subagent visibility