Add trusted-provenance admission gate to watch execution #1478

Parent: #1477

Goal

Add a deterministic provenance gate before an issue can trigger unattended watch execution.

Scope

• Extend work-item metadata to include issue author and GitHub �uthorAssociation or an equivalent trusted-provenance signal.
• Default unattended eligibility to trusted repository relationships such as OWNER, MEMBER, and COLLABORATOR.
• Hold external or unknown-provenance issues with a visible reason instead of executing them.
• Provide an explicit, auditable maintainer-controlled path for repositories that intentionally accept external triggers.
• Preserve --yolo for admitted trusted runs; do not add interactive approval prompts to the trusted path.
• Record admission decisions without copying raw issue content into audit logs.

Compatibility

• Trusted private/team repositories should continue using the same watch command with little or no behavior change.
• Public repositories will stop automatically executing outside-contributor issues unless explicitly admitted.
• This gate reduces exposure but does not address malicious or compromised trusted identities and does not by itself fix instruction injection.

Acceptance criteria

• Trusted and untrusted provenance cases have deterministic tests.
• Held issues report the reason and remain available for human triage.
• No untrusted issue reaches the privileged invocation before an explicit admission decision.
• --yolo behavior for admitted trusted runs is unchanged.
• Documentation explains migration and residual risk.