feat: add prerequisite gate and orphan check to check-issue skill

[isPANN]

Summary

• Rule Check 0 (Prerequisites): Before evaluating a [Rule] issue, verify both source and target problems are implemented in the codebase. If either is missing, fail with Blocked label and skip all remaining checks.
• Rule Check 3e (Code cross-validation): Cross-validate issue content against actual code implementations — fields, methods, objective direction, and size metric names.
• Model Check 1.5 (Orphan prevention): Verify at least one [Rule] issue references the model as source or target. If none found, fail with Orphan label.

These create a mutual dependency that ensures the reduction graph stays connected:

• [Rule] issues require both problems to be implemented → prevents rules referencing nonexistent problems
• [Model] issues require at least one [Rule] issue → prevents orphan nodes

Also updates flow diagram, report templates, and label application section.

Closes #264

Test plan

• Tested on [Model] issue [Model] GraphPartitioning #225 (GraphPartitioning) — passed all checks including orphan prevention (5 Rule issues found)
• Tested on [Rule] issue [Rule] KClique to BalancedCompleteBipartiteSubgraph #231 (Clique → BalancedCompleteBipartiteSubgraph) — correctly blocked (target not implemented)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.37%. Comparing base (52648df) to head (1faf39e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #265   +/-   ##
=======================================
  Coverage   96.37%   96.37%           
=======================================
  Files         200      200           
  Lines       27871    27871           
=======================================
  Hits        26861    26861           
  Misses       1010     1010           

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