feat(improve): hierarchical abstraction for lesson clusters

[itlackey]

Summary

Allow second-order abstraction so a cluster of lessons can roll up into a higher-order knowledge schema.

Scope

• Introduce a distinct second-order path instead of reusing the current single-lesson distill flow.
• Support lesson clusters producing one knowledge: schema.
• Preserve the existing first-order recursion guard.

Acceptance Criteria

• A related lesson cluster produces one knowledge: schema proposal.
• The doubled -lesson regression cannot reappear.
• Existing first-order distill tests still pass.

[itlackey]

Stays in 0.10.0. No split, but a 0.9 design constraint protects it: WS-3 restructures the distill/consolidate path, and per Part VI of docs/design/improve-reconciliation-plan.md that rework must not preclude a distinct second-order path and must preserve the first-order recursion guard — so this second-order abstraction won't require a second distill refactor, and the doubled--lesson regression can't reappear.