docs(readme): add comprehensive API coverage matrix

[platinummonkey]

Summary

Adds a comprehensive API coverage matrix to README.md showing what Datadog APIs are available vs implemented in pup, making it easy for users to understand feature coverage and gaps.

Changes

• Added collapsible API coverage section with 8 user-centric categories (README.md:18-157)
• Included all 33 implemented commands with current status
• Listed 20+ high-value unimplemented APIs across categories
• Added summary statistics: 33/85+ APIs (38.8% coverage)
• Status indicators: ✅ 23 Working, ⚠️ 7 Blocked, ⏳ 3 Planned, ❌ 52+ Not Implemented
• Mobile-friendly collapsible HTML sections with emoji icons
• Maintenance metadata: last updated date + API client version (v2.30.0)
• User-friendly features: search tip, GitHub Issues link, COMMANDS.md reference

Categories

1. 📊 Core Observability (5/9): Metrics, Logs, Traces, Events, RUM
2. 🔔 Monitoring & Alerting (6/9): Monitors, Dashboards, SLOs, Synthetics
3. 🔒 Security & Compliance (4/9): Security Monitoring, Vulnerabilities, Audit Logs
4. ☁️ Infrastructure & Cloud (5/8): Hosts, Tags, Network, AWS, GCP, Azure
5. 🚨 Incident & Operations (5/7): Incidents, On-Call, Error Tracking
6. 🔧 CI/CD & Development (1/3): CI Visibility
7. 👥 Organization & Access (4/6): Users, Organizations, API Keys
8. ⚙️ Platform & Configuration (4/6): Usage, Integrations, Pipelines

Benefits

• Users can quickly see what Datadog APIs are supported
• Clear visibility into coverage gaps and blocked features
• Easy navigation with collapsible sections
• Encourages feature requests via GitHub Issues
• Links to detailed command reference in COMMANDS.md

Testing

• Verified markdown rendering in GitHub preview
• Checked collapsible sections work correctly
• Validated all status symbols match COMMANDS.md
• Confirmed statistics accuracy (33 total, 23 working, 7 blocked, 3 planned)
• Tested mobile-friendly collapsible UI

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🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 76.8%

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:28: 94.1%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:26: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:20: 95.8%

## Summary

total:								(statements)		76.8%

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📈 Coverage Status: ❌ FAILED - Coverage below minimum threshold

_{Updated for commit 88d4b1a}