feat: add env vars for complete portable mode and isolation support

[Robitx]

Summary

• Add OPENCODE_DATA_DIR environment variable to override the data directory location
• Add OPENCODE_CACHE_DIR environment variable to override the cache directory location
• Add OPENCODE_LOG_DIR environment variable to override the log directory location
• Add OPENCODE_STATE_DIR environment variable to override the state directory location
• Enables complete portable mode when combined with OPENCODE_CONFIG_DIR
• Allows running multiple sandboxed/independent OpenCode instances on the same device
• Update documentation in cli.mdx, config.mdx, plugins.mdx, and troubleshooting.mdx

Use Cases

• Portable mode: Store all OpenCode data, cache, logs, and state alongside config in a single directory for easy sync across devices (e.g., USB drive, cloud storage)
• Isolated instances: Run multiple independent OpenCode instances on the same machine with completely separate data directories, caches, logs, and state
• Testing/development: Sandbox OpenCode for testing without affecting your main installation
• Custom deployments: Point all OpenCode storage to non-standard locations for containerized or restricted environments

Closes #4526 Closes #8968