feat(client): add AI agent detection to user agent

[jkirsteins]

Summary

Add automatic detection of AI coding assistants to the user agent string sent in HTTP requests to Datadog API.

Changes

• pkg/useragent/useragent.go - New package with user agent generation and AI agent detection
• pkg/useragent/useragent_test.go - Comprehensive tests with 100% coverage
• pkg/client/client.go - Use new useragent package, remove old getUserAgent() function
• pkg/client/client_test.go - Update tests to use useragent.Get()
• docs/ARCHITECTURE.md - Document user agent implementation

User Agent Format

Without AI agent:

pup/v0.1.0 (go go1.25.0; os darwin; arch arm64)

With Claude Code:

pup/v0.1.0 (go go1.25.0; os darwin; arch arm64) claude-code

With Cursor:

pup/v0.1.0 (go go1.25.0; os darwin; arch arm64) cursor

AI Agent Detection

Automatically detects AI agents via environment variables:

• CLAUDECODE=1 or CLAUDE_CODE=1 → appends claude-code
• CURSOR_AGENT=true or CURSOR_AGENT=1 → appends cursor
• Precedence: CLAUDECODE takes priority if both are set

Testing

• New pkg/useragent package: 100% test coverage
• All existing tests pass
• Manual testing with/without agent environment variables confirmed
• Integration test verifies user agent appears in HTTP headers

Architecture

Created dedicated pkg/useragent package (instead of keeping in pkg/client) for:

• Separation of concerns
• Reusability across packages
• Easier testing and mocking
• Future extensibility

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🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 80.4%

Threshold: 80% ✅

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:32: 94.4%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/useragent/useragent.go:32: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:20: 95.8%

## Summary

total:								(statements)		80.4%

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📈 Coverage Status: ✅ PASSED - Coverage meets minimum threshold

_{Updated for commit ff3cb6c}