Add AGENTS documentation for bundle and spec guidance

[ErwannRousseau]

Summary

• Add AGENTS.md documentation at the repository root and within src/Knp/DictionaryBundle and spec/.
• Capture project structure, conventions, anti-patterns, and verification commands for maintainers and contributors.

Details

• Document the runtime bundle layout, core dictionary flow, and where to make changes in src/Knp/DictionaryBundle/AGENTS.md.
• Document the spec layout, naming conventions, and coverage gaps in spec/AGENTS.md.
• Add top-level project knowledge base guidance in AGENTS.md for repository-wide conventions and commands.
• Keep instructions aligned with the existing bundle behavior, especially lazy dictionary evaluation and compiler pass ordering.