feat(apm): add APM Services/Entities API support

[majorgreys]

Summary

Implements 7 new APM commands for querying services, entities, dependencies, and service flow maps:

• pup apm services list - List all APM services
• pup apm services stats - Get service performance statistics
• pup apm services operations <service> - List operations for a service
• pup apm services resources <service> - List resources (endpoints) for a service+operation
• pup apm entities list - List APM entities with rich metadata
• pup apm dependencies list [service] - View service dependencies
• pup apm flow-map - Service flow map with performance metrics

All commands use the RawRequest pattern due to limited SDK support for APM APIs. The implementation follows patterns established in cmd/investigations.go for API calls outside the official SDK.

Test Plan

• All unit tests pass (go test ./cmd/...)
• Commands build successfully (go build)
• Integration tested against staging environment (datad0g.com)
• All 7 commands tested with various flag combinations (20+ test scenarios)
• Output formats verified (JSON, YAML, table)
• Error handling verified (missing flags, invalid parameters, auth errors)
• Documentation updated (README.md, COMMANDS.md, CHANGELOG.md)
• .envrc added to .gitignore to prevent committing staging credentials

Coverage

Test coverage matches existing command patterns: 77.9% on init functions, 0% on API call functions (consistent with other commands like monitors.go)

🤖 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 a40d11d}