[Change] Local GitHub hierarchy cache for OpenSpec planning

[djm81]

Why

OpenSpec and agent workflows still have to query GitHub ad hoc to rediscover Epics, Features, and their parent links before creating or syncing change issues. That is slow, expensive, and error-prone, especially now that planning hierarchy matters in both specfact-cli and specfact-cli-modules.

What Changes

• Add a deterministic repo-local hierarchy cache generator for GitHub 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 openspec/config.yaml and AGENTS.md to consult the cached hierarchy inventory first and rerun the sync script when fresh data is needed.
• Cover the script with tests so cache output and no-change behavior remain stable.

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

• agile-feature-hierarchy: Local governance 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 OpenSpec governance guidance in openspec/config.yaml and AGENTS.md.
• Affected workflow: OpenSpec change creation, GitHub issue creation/sync, and CHANGE_ORDER.md maintenance become cache-first instead of lookup-first.
• Cross-repo impact: a sibling change in specfact-cli-modules must implement the same pattern so both repos expose equivalent planning metadata locally.