feat(agent-registry): prefer globally installed agent binaries over npx

[ApolloS-Hub]

Summary

When a built-in agent adapter (e.g. claude-agent-acp, codex-acp) is globally installed and available on PATH, use it directly instead of falling back to npx package-exec.

Problem

The current agent resolution only checks:

1. Local node_modules install
2. npx package-exec bridge

In slow-network environments (high latency to npm registry, corporate proxies, China mainland, restricted networks), the npx download/cache-check during probe can time out, causing the acpx backend to stay at registered and never reach ready. This manifests as sessions_spawn(runtime="acp") hanging indefinitely.

Related: openclaw/openclaw#51345 — sessions_spawn(runtime="acp") hangs immediately

Solution

Add a new resolution step between local install and npx fallback:

1. Local node_modules install (existing)
2. Global PATH binary (NEW — via which/where)  ← added
3. npx package-exec bridge (existing fallback)

New resolveGlobalPathAgentLaunch() function uses which (Unix) or where (Windows) with a 5-second timeout to detect globally installed binaries.

Changes

• src/agent-registry.ts: Added resolveGlobalPathAgentLaunch() and integrated it into resolveBuiltInAgentLaunch() resolution chain
• src/acp/client.ts: Added spawn log message for global-path source
• test/agent-registry.test.ts: Added 4 new tests covering the global PATH resolution

Benefits

• Eliminates network dependency during agent spawn for users who pre-install adapters
• Faster startup in all environments (no npx cache check overhead)
• Zero breaking changes — existing behavior preserved as fallback
• Cross-platform: works on Unix (which) and Windows (where)

Test Results

All 16 tests pass, including 4 new tests:

• resolveGlobalPathAgentLaunch uses a globally installed binary from PATH
• resolveGlobalPathAgentLaunch returns undefined when binary is not on PATH
• resolveGlobalPathAgentLaunch ignores non-built-in commands
• resolveBuiltInAgentLaunch prefers global PATH over package-exec when local install unavailable