[Change] Local GitHub hierarchy cache for OpenSpec planning

[djm81]

Why

The modules repository now has its own Epic and Feature hierarchy, but contributors still have to query GitHub directly to rediscover parent Features and Epics before syncing OpenSpec changes. That creates unnecessary API traffic and makes cross-repo governance slower and less deterministic than it should be.

What Changes

• Add a deterministic repo-local hierarchy cache generator for specfact-cli-modules Epic and Feature issues.
• Persist a central markdown inventory under openspec/ with issue number, title, brief summary, labels, and hierarchy relationships.
• Add a lightweight fingerprint/state check so the sync exits quickly when Epic and Feature metadata has not changed.
• Update governance instructions in AGENTS.md for modules-side GitHub issue setup to consult the cache first and rerun sync only when needed.
• Keep the modules-side cache behavior aligned with the paired core change so both repos expose the same planning lookup pattern.

Capabilities

New Capabilities

• github-hierarchy-cache: Deterministic synchronization of GitHub Epic and Feature hierarchy metadata into a repo-local OpenSpec markdown cache for low-cost parent and planning lookups.

Modified Capabilities

• backlog-sync: Modules-side backlog and change-sync workflows must be able to resolve current Epic and Feature planning metadata from the repo-local cache before performing manual GitHub lookups.

Impact

• Affected code: new script and tests under scripts/ and tests/, plus governance guidance in AGENTS.md.
• Affected workflow: OpenSpec change creation and modules-side GitHub issue parenting become cache-first instead of lookup-first.
• Cross-repo impact: this change must stay aligned with specfact-cli so both repos use the same hierarchy-cache operating model.