training-agent: emit compact JWS governance_context with plan_hash

[bokelley]

Problem

The training agent currently emits an opaque UUID as governance_context (see server/src/training-agent/governance-handlers.ts:1040). The 3.0 spec (#2306, signed-governance-context changeset) requires governance agents to emit a compact JWS per the AdCP JWS profile, and #2455 now adds plan_hash as a required claim in that profile.

The training agent is the reference implementation the certification track points learners at. A learner reading docs/governance/campaign/tasks/check_governance.mdx sees the JWS contract; a learner stepping through the training agent sees a UUID. That gap bites both implementer onboarding (what does a real token look like?) and conformance (the training agent can't be used as a test vector source for the JWS profile).

Scope

Upgrade the training agent to emit a fully conformant compact JWS:

• Key management: issue a training Ed25519 keypair (alg: EdDSA), publish the JWKS at a well-known path reachable from the training harness, rotate-by-kid.
• Claims: iss, sub (plan_id), aud (seller URL from the caller context), iat, exp, jti (UUID v7), phase, caller, check_id, media_buy_id (conditional), policy_decision_hash (optional), and plan_hash (required per this PR's closed exclusion list).
• Canonicalization: plan_hash = base64url_no_pad(SHA-256(JCS(plan_payload))) with the excluded-fields list applied in code, not via convention.
• Revocation list: publish a signed governance-revocations.json at the training origin (empty body is fine; the presence is required so the conformance ramp-up tests can verify fetch-and-parse).
• Reference tests: golden-vector tests that round-trip the training agent's output through the reference TypeScript verifier in docs/building/implementation/security.mdx.

Why separate from #2455

#2455 was a spec-only change (claim definition). Upgrading the training agent to JWS is a larger implementation effort — new key material, new signing path, new verifier tests, new conformance storyboards. Rolling them together would have bloated the spec PR and delayed the ecosystem-signal that plan_hash is normative in 3.0.

Out of scope

• Production governance-agent key management (vendor concern).
• 3.1 step 16 seller verification (separate issue).