epic(cross-refs): complete and normalize cross-reference sections across all protocols and skills

[pentaxis93]

Summary

Cross-reference sections are the navigational tissue connecting protocols and
skills into a legible topology. Currently, coverage is uneven: two skills have
no cross-references at all, one protocol uses a non-standard section name, and
several protocols omit references to direct upstream or downstream consumers.
An implementer reading any single protocol should be able to see where it sits
in the system without cross-reading other files.

This epic brings every protocol and skill cross-reference section to the
standard established by begin, land, and document: present, accurately
described, and bidirectional where the dependency is bidirectional.

Task issues

Layer 0 — Add missing sections

• task(cross-refs): add cross-reference sections to reckon and research skills #188 — reckon, research — add cross-reference sections from scratch

Layer 1 — Normalize and fill gaps

• task(cross-refs): normalize and fill gaps in submit, verify, implement, specify, plan #189 — submit, verify, implement, specify, plan — normalize section name, add missing references

Layer 2 — Minor additions

• task(cross-refs): add reciprocal references to contract and decompose #190 — contract, decompose — add reciprocal references

Layer 0 (no deps):  #188
Layer 1 (no deps):  #189
Layer 2 (no deps):  #190

All three layers are independently shippable — no hard dependencies between
them. Layer numbering reflects priority order, not blocking order.

Acceptance criteria

• Every protocol and skill has a ## Cross-References section (not "Related Skills" or other variants)
• Every reference includes a one-line description of the relationship direction (what flows from A to B)
• No asymmetric gaps remain: if A references B, B references A (where the relationship is bidirectional)
• No new content is invented — descriptions reflect actual documented artifact/trigger relationships

[pentaxis93]

Closing: this epic misframed the survey finding as a cross-reference completeness problem. The actual finding — reckon needs to be an explicit ceremony element in decompose — is a structural ceremony-integration gap, not a documentation formatting gap. Addressed by #186. Legitimate cross-reference maintenance gaps (implement↔plan, decompose←survey, specify→contract, submit heading normalization) can be refiled accurately if pursued later.