fix(compliance): seed governance_agent_url default via storyboard-root context: block

[bokelley]

Closes #3913

Six governance storyboards (sync_governance steps in media-buy, brand-rights, and signal-marketplace scenarios) referenced $context.governance_agent_url in their sample_request payloads but never seeded it. Runners emitted SKIP: unresolved context variables from prior steps: governance_agent_url, causing the downstream steps to run without a registered governance binding and the storyboard to produce false negatives even against spec-correct agents. This PR adds a storyboard-root context: block to each affected YAML seeding the training-agent URL as the default, and formally adds the context: field to the top-level spec section in storyboard-schema.yaml (it was only described in the accumulator prose, not the field list). Narratives updated to remove the reference to a non-existent --governance-agent-url CLI flag.

Non-breaking justification: adds an optional context: block (new field, no wire change); all existing runners that do not implement accumulator source 2 simply ignore it — the SKIP behaviour is unchanged for those runners, which is the same as today. Runners that do implement source 2 (per the 3.0.5 spec) will now resolve the variable and exercise the step as intended.

Note — 3.0.x cherry-pick recommended: The dist/compliance/3.0.1–3.0.5 versioned snapshots are frozen and still carry the unresolved $context.governance_agent_url. Adopters pinned to a 3.0.x dist (including the bokelley/hello-adapters-gov-rights adapter that surfaced this issue at 3.0.5) need this cherry-picked to 3.0.x and a new patch release cut to see the fix in a versioned snapshot.

Sibling-repo-fix-needed: adcp-client — adcp storyboard run lacks a --context KEY=VALUE flag (single-step has --context JSON but full run mode does not). Adopters who need to override governance_agent_url to point at their own governance agent currently cannot do so without editing the storyboard YAML. The context: block default unblocks the training-agent path; the CLI flag is needed for the custom-agent path.

Pre-PR review:

• ad-tech-protocol-expert: approved — field placement correct, scalar-only constraint accurate, training-agent URL matches existing narratives; no blockers
• code-reviewer: approved — logic sound, all 6 storyboards patched, schema doc consistent, changeset correctly --empty; flagged 3.0.x frozen-dist gap (noted above)

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See #3121
for context.

Session: https://claude.ai/code/session_019r8L5mpimhpvn5NDBUbzjt

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