Emit a default-Copilot-configuration signal in lock file metadata

[salmanmkc]

Summary

The compiled lock file metadata (# gh-aw-metadata:) captures agent_id, schema_version, strict, agent_model, etc., but does not capture whether the Copilot engine is using the default GitHub configuration versus a custom provider / base URL / API target (BYOK).

Consumers of the compiled workflow (telemetry, dashboards, reporting) that want to classify "default Copilot config" runs versus custom-routed runs currently have to infer this from the compiled step env:, which is fragile:

• A custom engine.api-target is not visible in the step env at all — the host is embedded in the AWF config JSON inside the execution step's run: script, so env-based inference misses it entirely.
• GITHUB_COPILOT_BASE_URL also appears as a passthrough on the DIFC integrity-proxy step (via writeProxyUpstreamEnv) even for otherwise-default configurations, so naive "scan every step's env" inference produces false positives.
• Any env-based inference is coupled to the compiled step structure (step ids, which keys land where) and can silently drift when the compiler changes.

gh-aw already computes the authoritative answer at compile time, so it would be much more robust to surface it directly in the metadata.

Proposal

Add a boolean field to LockMetadata (e.g. engine_base_url_customized / copilot_default_config) populated from a shared predicate combining the signals the compiler already derives:

• isBYOKMode — COPILOT_PROVIDER_BASE_URL present in engine.env, or a non-GitHub engine.model-provider gateway.
• A non-empty GetCopilotAPITarget(...) — covers engine.api-target and a literal GITHUB_COPILOT_BASE_URL override.

Emit it in generateWorkflowHeader alongside the existing agent metadata.

Implementation pointers

• pkg/workflow/lock_schema.go — LockMetadata, AgentMetadataInfo, GenerateLockMetadata (add the field; bump schema version if desired — the field is additive/omitempty so older consumers ignore it).
• pkg/workflow/compiler_yaml_header.go — generateWorkflowHeader (populate the field; it already holds data / WorkflowData).
• pkg/workflow/copilot_engine_execution.go — isBYOKMode is currently inline; consider extracting a shared isCopilotBYOK/isCopilotDefaultConfig helper so the metadata value can't drift from the actual routing logic.
• pkg/workflow/engine_api_targets.go — GetCopilotAPITarget.
• Tests: pkg/workflow/lock_schema_test.go (TestGenerateLockMetadata*, TestLockMetadataToJSON), plus BYOK/api-target cases modeled on existing copilot_engine_test.go / engine_api_targets tests.

Benefit

Gives downstream consumers a single authoritative, drift-free signal that covers BYOK (model-provider + explicit engine.env) and engine.api-target, instead of fragile per-step env inference that cannot see api-target and is coupled to compiled output structure.