feat(skills): add shared task workflows

shared/skills/verification/facts-verification/SKILL.md

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+---
+name: facts-verification
+description: Verify factual claims against available evidence.
+---
+
+# Facts Verification
+
+Check important claims against source code, documentation, commands, or other appropriate evidence.
+
+Return the claim, evidence, and confidence. Mark unknowns explicitly.

shared/skills/verification/regression-verification/SKILL.md

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+---
+name: regression-verification
+description: Check that existing behavior was not unintentionally broken.
+---
+
+# Regression Verification
+
+Identify the affected existing behavior and run proportionate regression checks.
+
+Report results and any unverified areas. Do not claim checks that were not run.

shared/skills/verification/work-item-verification/SKILL.md

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+---
+name: work-item-verification
+description: Verify that a work item meets its stated acceptance criteria.
+---
+
+# Work Item Verification
+
+Compare the completed change with its requirements and acceptance criteria.
+
+Return passed criteria, failed criteria, checks run, and remaining limitations.