Add opaque private-contract loader and trusted event kickoff

[mark-e-deyoung]

Implement the next Phase-0 dispatcher experiment without expanding the public information surface.

Public-safe scope:

• add one generic registered task that tells Jules to load a private execution contract by opaque dispatch ID from the already-cloned approved target;
• define a strict public request-envelope schema containing only dispatch_id, opaque target alias, and generic task ID;
• add schema-only/non-secret validation for request envelopes;
• after Complete repository-level security controls for Jules dispatcher #1 administrative controls are verified, add a trusted default-branch event path that can dispatch a reviewed request envelope without requiring direct workflow_dispatch API access;
• keep real task text, repository/branch identity, artifact/source URLs, provider session metadata, result metadata, and performance records private;
• do not add a credential capable of writing private GitHub repositories to the public workflow;
• retain current provider-error/log minimization and no-untrusted-code invariants.

The first intended use is a deterministic evidence-acquisition task whose success/failure can be checked in its private target repository. Detailed target/task/provider/telemetry design is maintained in the private companion repository.

Dependency: do not activate the secret-bearing event trigger until #1 is complete/verified.

[mark-e-deyoung]

Implementation is now staged in draft PR #3. It adds the generic opaque private-contract-v1 transport: public workflow inputs remain an opaque target alias, registered task ID, and opaque correlation; Jules reads the real delegation/task records only after cloning the authorized private target. First real workload is tracked privately. Do not activate normal use until issue #1 administrative hardening is verified.