Add per-agent custom_env configuration that gets injected into the agent
subprocess at launch time. This enables users to configure custom API
endpoints (ANTHROPIC_BASE_URL), API keys (ANTHROPIC_API_KEY), and cloud
provider modes (CLAUDE_CODE_USE_BEDROCK, CLAUDE_CODE_USE_VERTEX) without
requiring code changes.
Changes:
- Migration 040: add custom_env JSONB column to agent table
- Backend: custom_env in agent CRUD API + claim endpoint
- Daemon: merge custom_env into subprocess environment variables
- Frontend: env var editor in agent settings (key-value pairs with
visibility toggle for sensitive values)
Closes#816
Related: #807, #809
