Add sealed public Actions execution lane

[mark-e-deyoung]

Adds a narrow public-safe execution adapter that uses standard GitHub-hosted Actions compute while keeping substantive result evidence out of Git history.

Key properties:

• originating/private authority decides whether a task is safe for public execution;
• public workflow receives only an opaque assignment, bounded compressed capsule, digest, timeout, and per-assignment age recipient;
• no private credentials or private-repository checkout;
• task stdout/stderr are captured into the result bundle instead of intentionally emitted to Actions logs;
• result bundle is encrypted with age and uploaded with a minimal receipt as a 1-day Actions artifact;
• public artifact is transport only, not project authority or durable result storage;
• dependency-free contract tests cover assignment/recipient validation, capsule digest checking, traversal rejection, and link rejection.

This does not alter the existing Jules dispatch path, add a scheduler/queue/database, or move project semantics into Agent Dispatch.