Improve agent package reliability and repository quality

[Maoxin1]

Summary

• make installation preflighted, idempotent, and safe for private local overlays
• add contract documentation and validators for all four agents
• add a unified repository validation suite and Windows CI
• add contribution, security, changelog, issue, PR, and dependency-management files

Validation

• ./tests/validate_repository.ps1
• installer clean install, forced update, private-overlay preservation, and collision preflight
• all four agent static validators and _invest unit test
• Markdown links, TOML/JSON, PowerShell parsing, and privacy-pattern checks

Notes

• behavior evaluations that can consume external services were intentionally not run
• existing Git history was not rewritten