fix(agent): install default system skills by agent id

[GatewayJ]

Summary

• Install embedded default system skills into the runtime skills directory resolved from the agent ID, not the display name.
• Update create/start regression coverage so the worker ID and display name differ, matching real typed-ID agent creation.

Root Cause

• feat(skillhub): add default system skills #235 introduced embedded system skills and default installation during agent create/start/recreate. That path installed skills using req.AgentName.
• feat(store): migrate local state to typed ids #245 migrated durable agent/workspace/runtime paths to immutable typed agent IDs. Runtime provisioning now materializes the agent home from AgentID, but the default system skill installer still used AgentName.
• For workers whose generated ID differs from their display name, system skills such as skill-installer and skill-creator could be copied to a sibling name-derived directory instead of the active runtime skills directory.

Fix

• Pass req.AgentID to installDefaultSystemSkills from runtime provisioning.
• Rename the helper parameter to agentID to reflect the storage contract.
• Adjust tests to use agent-worker-123 with display name alice, so the regression fails if ID/name paths drift again.

Validation

• env GOCACHE=/tmp/gocache go test ./internal/agent
• env GOCACHE=/tmp/gocache go test ./internal/agent ./internal/api