feat(ui): nest subagents in session lists

[edwin-zvs]

Summary

• persist parent_session_id on subagent session summaries and include subagents in session.list so clients can place them
• render subagents nested under their parent session in the TUI with default-expanded disclosure controls
• mirror the same nested/default-expanded subagent list behavior in the remote web UI

Tests

• cargo test -p agentd-cli subagents_render_under_parent_and_default_expanded -- --nocapture
• cargo test -p agentd list_includes_subagent_sessions_for_clients_to_nest -- --nocapture
• cargo check -p agentd-cli
• cargo check -p agentd -p agentd-adapter-zarvis -p agentd-mcp
• node --check /tmp/agentd-webui-check.js
• cargo build

Binaries

This PR changes crates/protocol, crates/daemon, crates/cli, crates/mcp, and crates/adapter-zarvis. Relevant binaries are agentd, agent, agentd-mcp, and agentd-adapter-zarvis. Full workspace built in debug profile:

• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agent
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd-mcp
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd-adapter-antigravity
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd-adapter-claude
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd-adapter-codex
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd-adapter-shell
• /Users/moon/agentd/.claude/worktrees/show-subagents-in-list/target/debug/agentd-adapter-zarvis