feat: make resume segment-index only

[guix4ever]

What changed

• Require every new Point to preserve the full Session with a complete native-Turn segment index.
• Make Resume traverse manifests only and return segment id, range, and title without historical records.
• Read original records only through an explicit Point segment reference.
• Remove the Full Resume product path and synchronize the bundled Skill and documentation.

Compatibility

• Point refs, DAG lineage, push/fetch, Commit Points, and custom metadata are unchanged.
• Existing v1/v2 Points remain readable through the hidden legacy compatibility path.
• New saves without segments now fail validation by design.

Validation

• 20 GitHub tests passed.
• Python compileall passed.
• git diff --check passed.