ci: implement ci pipeline

[pikann]

This pull request introduces new GitHub Actions workflows to automate CI checks for the backend, frontend, and MCP server components. Each workflow is tailored to its respective directory and includes steps for linting, building, type checking, and testing, improving code quality and reliability for pull requests.

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

• Adds a workflow that runs on backend-related pull requests, including linting with golangci-lint, building a WASM binary, and running Go tests with race detection and coverage reporting.

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

• Adds a workflow that runs on frontend-related pull requests, performing dependency installation, type checking, and building the production bundle using Bun.

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

• Adds a workflow that runs on MCP server pull requests, handling dependency installation, type checking, and building the MCP server with Bun.