fix(sync): improve pre-commit cache handling and enhance lessons structure

[diegoitaliait]

Description

Update pre-commit cache handling for better key generation. Enhance the syncing process for LESSONS.md to preserve target-authored content and align its structure. Rename and restructure the retained-learning ledger to LESSONS_LEARNED.md, updating all related references and sync processes.

Change Type

• Agent update
• Instruction update
• Prompt update
• Skill update
• Workflow/automation update
• Validation script update
• Documentation/configuration only
• Other

List of changes

Consumer Impact

• Affected customization assets:
• Backward compatibility notes:
• Rollout or migration notes (if any):

Testing Instructions

Validation Evidence

• Customization validator result:
• Stack-specific checks/tests result:
• Security/supply-chain checks result:

Breaking Changes

• This PR contains breaking changes

Checklist

• No secrets/tokens/credentials committed
• Repository artifacts remain in English
• validate-copilot-customizations.py --mode strict executed successfully
• Workflow action pins use full SHA with adjacent release/tag reference
• References remain repository-agnostic and reusable