fix(macos): scrub parent-session env, survive node upgrades and reboots, store lanes by real path (#22)

What broke (code audit, untested on a Mac):
- Without systemd the server is always spawned from inside a Claude Code
  session, so it inherited CLAUDECODE, CLAUDE_CODE_SESSION_ID,
  CLAUDE_PLUGIN_ROOT, ... and tmux handed them to every lane pane (a codex
  pane even resolved as `claude` in `agenthub`'s harness check).
- The ~/.local/bin/agenthub launcher pinned a versioned node path that
  `brew upgrade`/nvm deletes, leaving the CLI dead until the next bootstrap.
- A discovery entry left over from before a reboot could name a recycled pid,
  so the bootstrap's fast path believed a dashboard was live and
  stopDashboard could signal an unrelated process.
- Lanes were stored under the typed cwd; macOS reports /private/tmp and the
  on-disk case, so sessions never bound to a /tmp or differently-cased lane.

What changed:
- server/index.js deletes a DENYLIST of parent-session variables at startup;
  user config (CLAUDE_CONFIG_DIR, CLAUDE_CODE_USE_BEDROCK, ANTHROPIC_*) stays.
- linkCli's launcher falls back to `node` on PATH when the pinned one is gone.
- liveServers ignores entries whose startedAt predates this boot (60 s margin).
- createLane and POST /api/lanes/ensure store/look up fs.realpathSync.native
  of an existing cwd; the liveness reaper also tries the realpath. Existing
  rows are untouched.
- session-liveness gains an exec seam so the darwin ps+lsof branch is tested.
- plugin-doctor names ~/.zshrc on darwin; INSTALL.md gains a macOS section
  with the Mac verification checklist, mirrored in both READMEs and PLUGINS.md.
No launchd, no macOptionIsMeta, engines stays >=20.

server/lib/session-liveness.js

@@ -21,6 +21,13 @@ const fs = require("node:fs");
 const path = require("node:path");
 const { isInsideContainer } = require("../../scripts/install-hooks");
 
+let execImpl = execFileSync;
+
+/** Test seam — same pattern as tmux.js / desktop-env.js. */
+function __setExecImpl(fn) {
+  execImpl = fn || execFileSync;
+}
+
 const UNAVAILABLE = () => ({ available: false, cwds: new Set() });
 
 /**
@@ -62,7 +69,7 @@ function probeLiveCwds() {
 
   let psOut;
   try {
-    psOut = execFileSync("ps", ["-Ao", "pid=,args="], {
+    psOut = execImpl("ps", ["-Ao", "pid=,args="], {
       encoding: "utf8",
       timeout: 5_000,
       maxBuffer: 16 * 1024 * 1024,
@@ -95,7 +102,7 @@ function probeLiveCwds() {
   // machine format emits `p<pid>` / `f cwd` / `n<path>` records.
   let lsofOut;
   try {
-    lsofOut = execFileSync("lsof", ["-a", "-p", pids.join(","), "-d", "cwd", "-Fn"], {
+    lsofOut = execImpl("lsof", ["-a", "-p", pids.join(","), "-d", "cwd", "-Fn"], {
       encoding: "utf8",
       timeout: 10_000,
       maxBuffer: 16 * 1024 * 1024,
@@ -113,4 +120,4 @@ function probeLiveCwds() {
   return { available: true, cwds };
 }
 
-module.exports = { probeLiveCwds, isClaudeCommand };
+module.exports = { probeLiveCwds, isClaudeCommand, __setExecImpl };