fix(session): repair session skill patch merge

[ousugo]

Summary

• bind session-skill patch merging to the dedicated enabled session_skills schema and registry while preserving the external skills policy type
• materialize PATCH operations against existing SKILL.md content before producing policy updates
• preserve skill loader state, frontmatter metadata, and transaction leases throughout the Trainer → Optimizer → Updater → SkillProcessor write path
• add regression coverage for both new and existing skills using the real session-skill training pipeline

Root Cause

When session skill extraction produced a gradient, PatchMergePolicyOptimizer passed memory_type="skills" to PatchMergeContextProvider.

The provider resolved that name against the default memory registry, where the legacy skills schema is disabled. This caused:

ValueError: Memory schema not found or disabled: skills

before SkillPolicyUpdater could write the generated SKILL.md.

Session skills use a separate enabled session_skills schema and registry, so the optimizer now receives an explicit schema binding for that extraction model.

Validation

• Ruff passed for all changed Python files
• 15 targeted regression tests passed
• git diff --check upstream/main...HEAD passed
• merge-tree validation against the latest upstream/main completed without conflicts
• real new-skill and existing-skill Trainer → Optimizer → Updater → SkillProcessor paths are covered

Closes #4186