fix: clean up HTTP error messages for CLI commands

The new formatHttpError() utility:

• Maps common HTTP status codes (400, 401, 403, 404, 408, 429, 500, 502, 503, 504) to human-readable messages
• Detects and suppresses HTML content (Cloudflare error pages, etc.)
• Extracts message/error/detail fields from JSON error responses
• Truncates overly long plain text responses (>500 chars)

Test plan

• Verify build passes: bun run build
• Test with valid API key: postgresai issues list
• Test with invalid API key to verify 401 message formatting
• Simulate 502 error to verify HTML is not dumped

https://gitlab.com/postgres-ai/platform/-/issues/346