docs: define execution handoff

[dapi]

Summary

• define a governed, read-only Execution Handoff contract with lifecycle, schema, provenance, and non-goals
• distinguish declared priming context from observed execution, with a Feature Flow worked example
• add glossary, root/flow navigation, Feature priming reachability, and remove the conflicting Epic package-layer name

Validation

• ruby tools/validate-priming-manifests.rb template/memory-bank
• memory-bank-cli lint --scope-root template/memory-bank --entrypoint template/memory-bank/README.md
• memory-bank-cli doctor --profile template
• git diff --check
• codex review --uncommitted

Closes #95