Add Tidy First principles to AGENTS.md and CLAUDE.md

- Introduced "Tidy First" principles by Kent Beck to enhance code quality and maintainability.
- Added guidelines for separating structural and behavioral changes, and practical techniques for tidying code.
- Emphasized the importance of tidying before feature additions and bug fixes.

Author: Palbahngmiyine

AGENTS.md

@@ -78,6 +78,11 @@ SolapiMessageService → Fetcher (singleton) → Authenticator (static)
 - Full type hints on method params/returns
 - PHPDoc `@var`, `@param`, `@return`, `@throws` annotations
 
+**Tidy First (Kent Beck):**
+- Separate structural and behavioral changes into distinct commits
+- Tidy related code before making feature changes
+- Guard clauses, helper variables/functions, code proximity, symmetry normalization, delete unused code
+
 ## ANTI-PATTERNS
 
 - **Avoid catch-all nulls:** Many get* methods return `null` on any exception — check response validity

CLAUDE.md

@@ -52,6 +52,27 @@ SolapiMessageService → Fetcher (singleton) → Authenticator (static) → CURL
 - Default country code is "82" (Korea) in BaseMessage
 - Timezone hardcoded to Asia/Seoul in Authenticator
 
+## Tidy First Principles
+
+Follow Kent Beck's "Tidy First" principles when making code changes:
+
+**Core Principles:**
+- **Separate Structure from Behavior**: Separate structural changes (tidying) and behavioral changes (features) into distinct commits
+- **Tidy First**: Tidy related code before making feature changes to improve changeability
+- **Small Steps**: Keep tidying work completable within minutes to hours
+
+**Practical Techniques:**
+- Use guard clauses for early returns to eliminate nested if statements
+- Use helper variables/functions to clarify complex expressions
+- Keep related code physically close together
+- Express identical logic in identical ways (normalize symmetry)
+- Delete unused code immediately
+
+**When to Apply:**
+- Before adding new features, tidy the affected area
+- Before fixing bugs, clarify related code
+- During code review, identify tidying opportunities
+
 ## Important Behaviors
 
 - **Singleton State:** Fetcher singleton retains credentials - don't mix different API keys in the same process