FEAT-160: Add gateway server liveness detection and auto-recovery

[shafty023]

• Add alive flag, isAlive(), restart() to DesktopGatewayServer
• Fix stop() to handle stale (ERR_SERVER_NOT_RUNNING) handles gracefully
• Attach persistent error/close handlers with onUnexpectedClose callback
• Resolve actual OS-assigned port via server.address() in start()
• Extract GatewayRecoveryManager with single-flight restart, epoch guard, and gatewayHealthy flag into gateway-recovery.ts
• Wire recovery into DesktopApplication: liveness probe on cloud reconnect, auto-restart on unexpected close, gatewayHealthy in presence derivation
• Refactor onCloudSocketStatus to probe gateway before enabling executor
• Add serverAlive/gatewayHealthy to runtime status endpoint
• Switch all test files from fixed ports to OS-assigned port 0, eliminating EADDRINUSE failures when test files run concurrently
• Add 5 server-level liveness tests and 6 recovery state machine tests

Testing:

• All 323 tests pass with 0 failures
• Typecheck and lint clean

Risks:

• CloudSocketService still sends immediate presence on hello.ack before liveness probe completes (documented as known limitation, deferred)

https://app.closedloop.ai/implementation-plans/PLAN-86

[closedloop-ai-stage]

Code Review Summary

Status: Approved

Reviewers: Bug Hunter A (x3), Bug Hunter B, Unified Auditor, Premise Reviewer, Cloud Command Executor Specialist

Findings

Severity	Count
Blocking	0
High	0
Medium	1

MEDIUM Issues (consider)

1. [P3] [apps/desktop/src/main/gateway-recovery.ts:21] Deduplicated recoverGateway call silently drops second reason -- When a second recovery trigger fires while one is already in-flight, the new reason is silently discarded with no log entry. Add a warn log in the early-return branch.

Validation Stats

• Agent failures: 0 partitions skipped
• Cross-file grouped: 0 findings consolidated

Recommendation: Approve -- the gateway liveness detection and auto-recovery feature looks solid. One low-priority observability improvement suggested but not blocking.