fix: harden Claude ACP session creation stalls

[vincentkoc]

Summary

• add a Claude-specific ACP session creation timeout so acpx does not hang forever on stalled session/new calls
• surface an actionable CLAUDE_ACP_SESSION_CREATE_TIMEOUT error with non-interactive guidance for harness users
• add mock-agent and integration coverage for a hung Claude ACP adapter

Validation

• pnpm run check
• live smoke: acpx claude sessions new --name claude-live-smoke completed on macOS arm64, but took ~46s, so the built-in guard is set to 60s

Fixes: #29