refactor(specs): refine runner image spec for format compliance

Strip implementation details (Design Decisions, Implementation Touchpoints,
Example Dockerfile, internal function names, concrete CMD strings). Rename
Overview to Purpose. Fold init-hooks rationale into Purpose paragraph.
Align RBAC with endpoint-level model. Simplify all requirements to
observable behavior only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jbpratt