Add CLAUDE.md for Claude Code guidance

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+toolhive-core is a shared Go library providing stable, well-tested utilities for the ToolHive ecosystem. It serves as a common dependency for toolhive, dockyard, toolhive-registry, and toolhive-registry-server projects.
+
+## Build and Development Commands
+
+This project uses [Task](https://taskfile.dev/) for automation:
+
+```bash
+task              # Run all checks (lint + test)
+task lint         # Run golangci-lint and go vet
+task test         # Run tests with race detection
+```
+
+Run a single test:
+```bash
+go test -race -run TestFunctionName ./path/to/package
+```
+
+Other commands:
+```bash
+task lint-fix      # Auto-fix linting issues
+task test-coverage # Run tests with coverage report
+task gen           # Generate mocks using mockgen
+task tidy          # go mod tidy && go mod verify
+task license-check # Verify SPDX license headers
+task license-fix   # Add missing license headers
+```
+
+## Development Workflow
+
+1. Implement changes with tests (≥70% coverage)
+2. Run `task` to verify lint + tests pass
+3. If adding mocks: add `//go:generate` directive, run `task gen`
+4. Run `task license-check` before committing
+
+## Boundaries
+
+### Never
+- Remove or skip existing tests
+- Change exported interface signatures without discussion
+- Modify `go.mod` dependencies without being asked
+
+### Ask First
+- Adding new packages (need to consider stability track)
+- Adding new dependencies
+- Changes to error codes in `httperr`
+
+### Always
+- Run `task` before considering work complete
+- Include table-driven tests for new functions
+- Use interface-based design for new packages (see `env.Reader` pattern)
+
+## Architecture
+
+### Package Design Principles
+
+1. **Interface-based design** - Public APIs expose interfaces for testability and mock generation
+2. **Dependency injection** - Enables mocking (see `env.Reader` interface pattern)
+3. **Error wrapping** - Uses Go 1.13+ error wrapping compatible with `errors.Is()` and `errors.As()`
+4. **Security-first validation** - All validators check for injection attacks (CRLF, control chars) and enforce length limits
+
+### Current Packages
+
+| Package | Purpose |
+|---------|---------|
+| `env` | Environment variable abstraction with `Reader` interface for testable code |
+| `httperr` | Wrap errors with HTTP status codes; use `WithCode()`, `Code()`, `New()` |
+| `validation/http` | RFC 7230/8707 compliant HTTP header and URI validation |
+| `validation/group` | Group name validation (lowercase alphanumeric, underscore, dash, space) |
+
+### Mock Generation
+
+Mocks are generated using `go.uber.org/mock`. Add `//go:generate` directives in source files, then run `task gen`.
+
+## Code Quality Notes
+
+- **Test coverage**: ≥70% required for package graduation
+- **License headers**: Run `task license-check` before committing to verify SPDX headers
+
+## Stability Guarantees
+
+Packages follow graduation tracks:
+- **Stable**: Production-ready, semantic versioning applies
+- **Beta**: API may change with deprecation notices
+- **Alpha**: Experimental, breaking changes possible