chore: add .editorconfig (attributes on their own line) + code-style rules

Shared Rider/ReSharper formatting via .editorconfig — resharper_place_attribute_on_same_line=never
so attributes never pack onto the member or onto each other. CONTRIBUTING documents the comment
convention (XML / WHY: / TODO: / HACK:, strip obvious what-comments) and a before-you-commit
checklist: reformat, tests, changelog, lockstep version bump.

Author: NeoXider

.editorconfig

@@ -0,0 +1,32 @@
+root = true
+
+[*]
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.cs]
+indent_style = space
+indent_size = 4
+
+# ReSharper / Rider formatting shared with the whole team.
+# Attributes ALWAYS on their own line — never packed onto the member or onto each other
+# (e.g. never `[SerializeField] private int x;` or `[Tooltip("...")] [SerializeField]`).
+resharper_place_attribute_on_same_line = never
+resharper_csharp_place_attribute_on_same_line = never
+resharper_max_attribute_length_for_same_line = 0
+
+# Keep other members from being collapsed onto one line.
+resharper_place_accessorholder_attribute_on_same_line = never
+resharper_place_field_attribute_on_same_line = never
+resharper_place_type_attribute_on_same_line = never
+resharper_place_method_attribute_on_same_line = never
+
+[*.{json,uxml,uss,md,yml,yaml}]
+indent_style = space
+indent_size = 2
+
+# Unity-generated files: do not fight Unity's own formatting.
+[*.{meta,asset,prefab,unity,asmdef,mat}]
+trim_trailing_whitespace = false
+insert_final_newline = false

CONTRIBUTING.md

@@ -38,3 +38,30 @@ secrets: `UNITY_LICENSE`, `UNITY_EMAIL`, `UNITY_PASSWORD`.
 
 New Lua-dependent test files must be wrapped in `#if !COREAI_NO_LUA` so the
 `no-lua` job compiles.
+
+## Code style
+
+Formatting is shared through the repo `.editorconfig` (Rider/ReSharper read it directly). Run
+**Rider → Code → Reformat & Cleanup Code** before committing so a diff never mixes logic with style.
+Notably: **every attribute goes on its own line** — never `[SerializeField] private int x;` and never
+`[Tooltip("...")] [SerializeField]` packed together.
+
+### Comments
+
+Keep only comments that carry information the code cannot. Label the intentional kinds with an explicit
+prefix and delete everything else:
+
+- `/// <summary>` — XML docs on public/protected API.
+- `// WHY:` — rationale, non-obvious invariants, ordering constraints, workarounds, external-format notes.
+- `// TODO:` — deferred work.
+- `// HACK:` — deliberate shortcuts worth flagging.
+
+Remove obvious "what" comments that merely restate the next line, step-number narration (`// 1. Create…`),
+and section-divider banners.
+
+## Before you commit (checklist)
+
+1. **Reformat** — Rider *Reformat & Cleanup Code* (applies `.editorconfig`, incl. attribute-per-line).
+2. **Tests** — run the EditMode gate green (CI runs `lua` + `no-lua`; run locally in batchmode for a fast pre-push check).
+3. **Changelog** — add an entry under `[Unreleased]` (or the release section) in the affected package `CHANGELOG.md`.
+4. **Version** — for a release, bump **all five** `package.json` files in lockstep to the same version.