Resolve planning artifacts from parent issues

[rochecompaan]

Part of #191.

Follow-up to #180.

Blocked by: #206

Goal

Allow an Issue Run to resolve each required specification and plan from either the current issue or its parent issue without copying or duplicating planning artifacts.

Scope

• Define a provider-neutral artifact binding for specification and plan sources.
• Support self and parent independently for each artifact.
• Define and document how bindings are declared on an issue.
• Resolve parent through the host issue relationship, then save the exact owner issue identity.
• Resolve each required artifact to a stable path and commit or merge revision before it is consumed.
• Persist the resolved owner, path, revision, and relevant approval provenance in durable run state.
• Create missing required artifacts only when their source is self.
• Block with a clear result when a required parent artifact, parent relationship, or required review provenance cannot be resolved safely.
• Reuse a valid parent artifact review instead of creating an empty or duplicate planning pull request.
• Keep resume behavior stable when the parent issue or its artifacts change after the child run starts.
• Add GitHub and Forgejo behavior tests, including unsupported or ambiguous parent relationships.

Out of scope

• Searching arbitrary ancestors.
• Implicit fallback from parent to self.
• Copying inherited artifacts into the child issue.
• Coordination-only Issue Runs.
• Binding artifacts to arbitrary unrelated issues.

Acceptance criteria

• Specification and plan sources can each be set independently to self or parent.
• A child can use an approved parent specification and create its own plan.
• A child can use a parent-owned plan when the binding identifies a valid applicable artifact.
• Parent-owned artifacts are not regenerated or republished as child-owned artifacts.
• Resolved bindings include stable ownership and revision information in run state.
• Resume continues with the pinned artifact revision even if the parent later changes.
• Missing, unsupported, stale, or ambiguous parent bindings fail closed with actionable diagnostics.
• Provider contract and scenario tests cover GitHub and Forgejo behavior.