docs: enshrine testability as core development practice

Add comprehensive documentation and tooling to make testability
the default way of working:

Documentation:
- TESTABILITY.md: Complete guide with principles, examples, checklists
- FEATURE_TEMPLATE.md: Step-by-step template for new features
- Updated CONTRIBUTING.md with testability section
- PR template with testability checklist

Key principles enshrined:
1. Extract pure functions to utils/ modules
2. 100% coverage on utilities
3. No non-null assertions (!)
4. Organize by domain, not "misc"
5. Atomic commits with clear dependencies

Coverage targets:
- Pure utilities: 100%
- Integration: >80%
- CLI/UI: >60%

Scripts:
- Added test:coverage command for easy coverage checks

Real examples referenced:
- Explorer subagent (99 tests, 100% utils coverage)
- Repository indexer (87 tests, 100% utils coverage)

This establishes testability as a first-class concern and provides
clear guidance for all future development.

Author: prosdev

.github/pull_request_template.md

@@ -0,0 +1,73 @@
+## 📋 Description
+
+<!-- Briefly describe what this PR does and why -->
+
+## 🔗 Related Issues
+
+<!-- Link to related GitHub issues -->
+Closes #
+
+## 🧪 Testability Checklist
+
+<!-- Check all that apply -->
+
+- [ ] Pure functions extracted to `utils/` modules
+- [ ] Utilities achieve 100% coverage (statements & functions)
+- [ ] No `!` non-null assertions (use guard clauses or optional chaining)
+- [ ] Modules organized by domain (not generic "utils")
+- [ ] Each module < 200 lines
+- [ ] Atomic commits with clear dependencies
+
+## ✅ Testing
+
+<!-- Describe testing approach -->
+
+- [ ] Unit tests added/updated
+- [ ] Integration tests added/updated
+- [ ] Coverage meets targets:
+  - Pure utilities: 100%
+  - Integration: >80%
+  - CLI/UI: >60%
+- [ ] All tests passing locally
+
+## 📊 Coverage
+
+<!-- Paste coverage report for changed files -->
+
+```
+Before: X% statements, Y% branches, Z% functions
+After:  X% statements, Y% branches, Z% functions
+```
+
+## 🏗️ Architecture
+
+<!-- If refactoring, describe the structure -->
+
+- [ ] Follows dependency order (foundation → dependent → integration)
+- [ ] Barrel exports for clean imports
+- [ ] Clear separation of pure/impure code
+
+## 📝 Documentation
+
+- [ ] Updated README if public API changed
+- [ ] Added JSDoc for public functions
+- [ ] Updated CHANGELOG (if applicable)
+
+## 🚀 Deployment
+
+- [ ] No breaking changes
+- [ ] Backward compatible
+- [ ] Database migrations (if applicable)
+
+## 📸 Screenshots
+
+<!-- If UI changes, add screenshots -->
+
+---
+
+**Commit Strategy:**
+<!-- Did you follow granular commits? -->
+- [ ] Atomic commits (each builds independently)
+- [ ] Conventional commit messages
+- [ ] Clear commit descriptions
+

CONTRIBUTING.md

@@ -1,6 +1,13 @@
-# Contributing to TS Monorepo
+# Contributing to Dev-Agent
 
-Thank you for considering contributing to our TypeScript monorepo! This document outlines the process for contributing and the standards we follow.
+Thank you for considering contributing to dev-agent! This document outlines the process for contributing and the standards we follow.
+
+## 🎯 **Core Values**
+
+1. **Testability First** - If it's hard to test, refactor it
+2. **Modularity** - Small, focused, reusable modules
+3. **100% Coverage on Utilities** - Pure functions should be fully tested
+4. **Atomic Commits** - Each commit should build and test independently
 
 ## Development Process
 
@@ -46,11 +53,56 @@ feat(core): add new API method for authentication
 3. Add the package to relevant workspace configurations.
 4. Update path mappings in the root `tsconfig.json`.
 
-## Testing
+## Testing & Testability
+
+### 📖 **Read First:** [TESTABILITY.md](./docs/TESTABILITY.md)
+
+Our comprehensive testability guide covers:
+- When and how to extract utilities
+- Organization patterns
+- Coverage targets
+- Real-world examples
+
+### **Quick Rules:**
+
+1. **Extract Pure Functions** to `utils/` modules
+   - ✅ DO: `utils/formatting.ts` with `formatDocument(doc: Document)`
+   - ❌ DON'T: Private methods in 500-line classes
+
+2. **Aim for 100% on Utilities**
+   - Pure functions are easy to test
+   - No mocks needed
+   - Foundation for everything else
+
+3. **No Non-Null Assertions (`!`)**
+   - Use guard clauses or optional chaining
+   - Makes code safer and more testable
+
+4. **Organize by Domain**
+   - ✅ `utils/strings.ts`, `utils/dates.ts`, `utils/validation.ts`
+   - ❌ `utils.ts` (500 lines of everything)
+
+### **Coverage Targets:**
+
+| Code Type | Target | Example |
+|-----------|--------|---------|
+| **Pure Utilities** | 100% | `formatDocument()`, `calculateCoverage()` |
+| **Integration** | >80% | `RepositoryIndexer`, `ExplorerAgent` |
+| **CLI/UI** | >60% | Command handlers, spinners |
+
+### **Before Submitting:**
+
+```bash
+# Run tests with coverage
+pnpm vitest run --coverage
+
+# Check specific package
+pnpm vitest run packages/core/src/indexer --coverage
+```
 
-- Write tests for all new features and bug fixes.
-- Run existing tests to ensure your changes don't break existing functionality.
-- Aim for good test coverage.
+- Write tests for all new features and bug fixes
+- Run existing tests to ensure your changes don't break existing functionality
+- See [TESTABILITY.md](./docs/TESTABILITY.md) for detailed guidelines
 
 ## Code Style