feat: add online external runtime channel

[pilprod]

Summary

• route lifecycle and A2A calls through one explicit persisted runtime backend registry
• add an authenticated outbound-client reverse HTTPS broker for Codex and Claude slots
• wire the broker into controller-v2 behind a default-disabled, fail-closed feature gate on a separate device listener
• require one explicit device identity, file-backed bearer token, and explicit Codex and/or Claude slot IDs
• fence duplicate device sessions with monotonic generations and distinguish offline-before-dispatch from unknown-outcome-after-dispatch
• persist immutable backend identity on runtime revisions with migration 000018
• validate A2A v1 compatibility on Create/Resume and return a typed unsupported-checkpoint result for external runtimes
• bound paths, bodies, headers, token reads, timeouts, and concurrency while preventing user, Temporal, cookie, tracing, and hop-by-hop headers from crossing to a client device

Feature gate

• disabled by default with EXTERNAL_GATEWAY_ENABLED=false; token and placement settings are not read
• enabled mode requires EXTERNAL_GATEWAY_TOKEN_FILE, EXTERNAL_GATEWAY_DEVICE_ID, and at least one slot ID
• the in-memory broker listens separately on EXTERNAL_GATEWAY_BIND_ADDRESS (default :8085)
• the controller must remain single-replica until session ownership is externalized

Verification

• go test -race ./core/cmd/controller-v2 ./core/v2/runtimebackend ./core/v2/externalruntime ./core/v2/externalgateway ./core/v2/a2agateway ./core/v2/agentinstance ./core/v2/checkpoint ./core/v2/substrate
• go vet ./core/cmd/controller-v2 ./core/v2/runtimebackend ./core/v2/externalruntime ./core/v2/externalgateway ./core/v2/a2agateway ./core/v2/agentinstance ./core/v2/checkpoint ./core/v2/substrate ./core/internal/database
• go test ./core/pkg/migrations -run '^TestMigrationGuards$'
• make -C go sqlc-generate
• git diff --check origin/feature/external-runtime-gateway-v3...HEAD

PostgreSQL-backed migration and database integration tests could not run locally because rootless Docker is not installed on this workstation.

Stack

Depends on #6. This is intentionally an online, single-replica in-memory transport slice. Durable offline dispatch, multi-replica broker coordination, automated device enrollment, and external Harness compilation are not claimed here.