feat: detect agents hosted in editor-embedded terminals

[devgony]

Summary

Agents launched from nvim/vim :terminal buffers (e.g. opencode or codex started by an nvim plugin) run on their own pty and process group, so they never join the pane's foreground job and were invisible to herdr's process-based agent identification on Unix. Windows already finds them via descendant traversal; this brings Unix to parity.

Changes

• platform::descendant_processes(root_pid): bounded (64) BFS descendant walk — macOS proc_listpids(PROC_PPID_ONLY), Linux /proc/*/task/*/children, Windows snapshot reuse, fallback stub
• detect::identify_agent_hosted_by_editor: when the foreground job contains a terminal-hosting editor (nvim/vim/vi) and no agent was identified, identify agents among the editor's descendants using the existing scoring (including runtime-wrapped agents)
• Wired as a last-resort fallback in the pane detection probe and live_runtime_agent; descendants are scanned only when an editor is foreground and nothing else matched, so normal panes pay no extra cost

Testing

• Unit tests: editor-hosted identification (direct, runtime-wrapped, no-agent descendants, non-editor skip) and probe fallback wiring
• Live tests: descendant_processes finds a spawned child on macOS and Linux
• Manual (macOS, debug session): nvim -c ':terminal codex' → pane reports agent: codex, detected through nvim → nvim --embed → agent; clears after the agent exits while nvim keeps running, and when nvim exits
• cargo nextest green; cargo clippy -D warnings clean on host and x86_64-pc-windows-msvc; docs/next changelog updated

[kangal-bot]

Hi @devgony, thanks for your interest in contributing.

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