fix: clean workspace + single CI + wasm build

[nomirayuki]

Summary

• Cleaned up workspace structure to follow Cargo standards.
• Removed duplicate 'wasmtime' dependency causing build errors.
• Reduced CI workflows to a single file (ci.yml) for clarity and faster builds.
• Added wasm32-unknown-unknown build target for plugin to run in WASP runtime.
• Centralized dependency versions with workspace.dependencies.

Changes

• Rebuilt root Cargo.toml with workspace configuration.
• Added agent/host and agent/plugin with initial structure and files.
• Created a single GitHub Actions workflow for formatting, linting, and wasm build.
• Removed old/duplicate CI files.
• Verified successful local build.

Motivation

Ensures the repo is ready for:

• UOMI Core Contributor contributions.
• Smooth CI integration and deployment.
• Fewer build errors on GitHub Actions.

Test Plan

• Local build in Codespaces passed.
• ci.yml workflow runs successfully on branch fix/workspace-ci.