macOS support for live session (in-use) detection

[davidtaing]

Summary

treetop builds and runs on macOS today, but its automatic in-use detection is Linux-only. On macOS, worktrees with a live agent session running inside them are not marked ● unless the .treetop-inuse marker file is present.

Background

In-use detection has two independent signals (see session.go):

1. .treetop-inuse marker file (marker.go) — cross-platform and deterministic. Already works on macOS.
2. /proc session scan (session.go scanSessions) — best-effort auto-detection of live agent sessions. It walks /proc/<pid> to record, per agent process:
   • its cwd (catches a top-level claude running in a worktree)
   • the regular files it holds open (catches in-process subagents that never chdir into the worktree but read/write its files)

scanSessions() early-returns supported: false when runtime.GOOS != "linux" (session.go:31), and supportsProcScan() (main.go:158) reports it unsupported. So on macOS only signal (1) fires.

Goal

Implement a darwin equivalent of the /proc scan so live agent sessions are auto-detected on macOS without requiring the marker file — matching the Linux experience (top-level sessions and in-process subagents).

Approach

macOS has no /proc. Equivalent process introspection is available via libproc:

• enumerate pids — proc_listpids / proc_listallpids
• cwd — proc_pidinfo(pid, PROC_PIDVNODEPATHINFO, ...) -> vnodePathInfo.vip_path
• open files — proc_pidinfo(pid, PROC_PIDLISTFDS, ...) then proc_pidfdinfo(..., PROC_PIDFDVNODEPATHINFO, ...) per fd to resolve regular-file paths

Either via cgo against <libproc.h> or by shelling out to lsof as a fallback. cgo/libproc is preferred (no external dependency, faster, no per-pid process spawn).

Suggested structure

• Split the platform-specific scan behind a build tag: session_linux.go (existing /proc logic) and session_darwin.go (new libproc logic), with the shared sessionScan struct and the agent-matching / decay-tracker logic staying platform-neutral.
• isAgentProcess and resolvedLink/openFiles helpers need darwin counterparts.
• Update supportsProcScan() / the help text in main.go:37 so the "Linux-only" caveat reflects darwin support.

Acceptance criteria

• A top-level claude/agent session running in a worktree is marked ● on macOS with no marker file.
• An in-process subagent (open files only, no cwd match) is detected on macOS.
• Linux behaviour is unchanged.
• Help text and README "in-use detection" notes updated for macOS.
• Tests cover the darwin path where feasible (or the platform-neutral matching logic stays unit-tested).

Out of scope

• Windows support.