fix(agent): give launchd units a log sink (StandardOut/ErrorPath)

launchd has no default log destination (unlike systemd's journald), so a unit
that fails to start (bare node, missing env) was silent — which hid the EX_CONFIG
+ loadConfig failures during this deploy. Route both units' stdout+stderr to
<stateDir>/{base-app,agent}.log. 281 tests pass.

agent/src/service/launchd.ts

@@ -78,6 +78,7 @@ export function buildLaunchdPlist(
   programArguments: readonly string[],
   env: ServiceEnv = {},
   label: string = AGENT_LABEL,
+  logPath?: string,
 ): string {
   return [
     '<?xml version="1.0" encoding="UTF-8"?>',
@@ -91,6 +92,16 @@ export function buildLaunchdPlist(
     '  <true/>',
     '  <key>KeepAlive</key>',
     '  <true/>',
+    // launchd's default has no log sink (unlike systemd's journald), so a crashing unit is silent.
+    // Route stdout+stderr to a file so `pair --install` failures are diagnosable out of the box.
+    ...(logPath !== undefined
+      ? [
+          '  <key>StandardOutPath</key>',
+          `  <string>${escapeXml(logPath)}</string>`,
+          '  <key>StandardErrorPath</key>',
+          `  <string>${escapeXml(logPath)}</string>`,
+        ]
+      : []),
     ...environmentVariablesBlock(env),
     '</dict>',
     '</plist>',