docs: fold stocktake rule guidance into common rules

rules/common/coding-style.md

@@ -12,6 +12,26 @@ CORRECT: update(original, field, value) → returns new copy with change
 
 Rationale: Immutable data prevents hidden side effects, makes debugging easier, and enables safe concurrency.
 
+## Core Principles
+
+### KISS (Keep It Simple)
+
+- Prefer the simplest solution that actually works
+- Avoid premature optimization
+- Optimize for clarity over cleverness
+
+### DRY (Don't Repeat Yourself)
+
+- Extract repeated logic into shared functions or utilities
+- Avoid copy-paste implementation drift
+- Introduce abstractions when repetition is real, not speculative
+
+### YAGNI (You Aren't Gonna Need It)
+
+- Do not build features or abstractions before they are needed
+- Avoid speculative generality
+- Start simple, then refactor when the pressure is real
+
 ## File Organization
 
 MANY SMALL FILES > FEW LARGE FILES:
@@ -36,6 +56,28 @@ ALWAYS validate at system boundaries:
 - Fail fast with clear error messages
 - Never trust external data (API responses, user input, file content)
 
+## Naming Conventions
+
+- Variables and functions: `camelCase` with descriptive names
+- Booleans: prefer `is`, `has`, `should`, or `can` prefixes
+- Interfaces, types, and components: `PascalCase`
+- Constants: `UPPER_SNAKE_CASE`
+- Custom hooks: `camelCase` with a `use` prefix
+
+## Code Smells to Avoid
+
+### Deep Nesting
+
+Prefer early returns over nested conditionals once the logic starts stacking.
+
+### Magic Numbers
+
+Use named constants for meaningful thresholds, delays, and limits.
+
+### Long Functions
+
+Split large functions into focused pieces with clear responsibilities.
+
 ## Code Quality Checklist
 
 Before marking work complete: