ci: implement ci pipeline

[pikann]

This pull request introduces dedicated GitHub Actions CI workflows for the backend, frontend, and MCP server components. Each workflow is tailored to its respective directory and ensures code quality through linting, type checking, building, and testing as appropriate.

New CI workflows:

Backend CI pipeline (.github/workflows/backend-pr-ci.yml):

• Adds a workflow that runs linting (golangci-lint), builds the backend (including a WASM build), and executes tests with coverage reporting for all pull requests affecting the backend directory.

Frontend CI pipeline (.github/workflows/frontend-pr-ci.yml):

• Introduces a workflow to typecheck and build the frontend using Bun, with dependency caching for faster runs, triggered on pull requests to the frontend directory.

MCP server CI pipeline (.github/workflows/mcp-pr-ci.yml):

• Adds a workflow for the MCP server that typechecks and builds the codebase using Bun, with caching for dependencies, on pull requests to the mcp directory.