fix(compliance): default governance_agent_url to seller-local stub (#5665) [Option B — alternative]

[danyliukmykola]

Alternative to the Option A PR for #5665, per the triage's Option B. Opened for completeness so the A-vs-B call can be made against concrete diffs.

Defaults governance_agent_url to a seller-served stub (the seller's own comply_test_controller), removing the counterparty-supplied-URL egress/SSRF surface.

Two caveats vs Option A:

1. It keeps the scenario mandatory — it does not create the not_applicable opt-out the issue title asks for. Sellers still must implement outbound governance consultation (now against a local stub instead of an external agent).
2. It depends on the runner exposing a controller-self-URL placeholder — {{runner.controller_url}} doesn't exist yet (only {{runner.webhook_url:…}} and {{prior_step.…}} are runner-managed today), so this spans adcp-client too, not just scenario YAML.

We'd lean Option A — self-contained, and it actually creates the opt-out.

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[bokelley]

Closing in favor of Option A (#5676), now merged.

Recap of why A over B, after independent verification of the runner:

• B isn't shippable today — {{runner.controller_url}} exists nowhere in @adcp/sdk (the only runner placeholders are {{runner.webhook_base}} and {{runner.webhook_url:<step>}}). It would emit a literal unresolved token.
• B keeps the scenario mandatory — it never creates the not_applicable opt-out the issue asks for.
• B doesn't remove the egress — it re-defaults governance_agent_url to a self-URL, but the outbound check_governance call still happens and an explicit --context override is still honored, so the counterparty-URL path remains.

The SSRF concern B was reaching for is real and tracked separately as a docs follow-up (governance_agents[].url missing from the security.mdx SSRF enumeration).