Rename docs to lowercase snake_case, merge multi-combo, and add suit following refinement proposal

[ejfn]

This PR addresses documentation cleanups and introduces a new proposal:

1. Renames all files in docs/ to lowercase snake_case (e.g., ai_system.md, game_rules.md, versioning.md).
2. Merges the contents of docs/MULTI_COMBO.md into docs/game_rules.md.
3. Prefixes proposal filenames with the leading yyyy-mm-dd_ date format.
4. Adds the new proposal docs/proposals/2026-06-01_suit_following_refinement.md for strategic suit following.
5. Updates index and file references in AGENTS.md and README.md.