Contract Development Workflow Issues and Missing Documentation

## Summary
While developing custom contracts with DevKit, I've encountered several workflow issues and gaps in documentation that make contract development more difficult than it should be. These issues primarily relate to dependency management, build processes, and the lack of fast feedback loops for contract compilation.

## Issues Identified

### 1. Missing Documentation for Adding Contract Dependencies

**Problem**: When developers need to install libraries or packages for custom contracts, there's no clear documentation on how to properly add dependencies.

**Current Workaround Required**:
- For Forge-based dependencies:
1. Navigate to `.devkit/contracts/` and use `forge install `
2. Update remappings in `.devkit/contracts/foundry.toml` if needed
- For Node.js-based dependencies:
1. Create a `package.json` file in `.devkit/contracts/`
2. Update remapping in `.devkit/contracts/foundry.toml`
3. Manually modify the build command in `.devkit/scripts/build` to:
```bash
cd .devkit/contracts && npm install && forge clean && forge build -- --include ../../contracts/**/*.sol >&2
```

**Impact**: This creates a poor developer experience and requires deep knowledge of the DevKit internals.

### 2. Build and Test Process Doesn't Include Custom Contracts Directory

**Problem**: Both the `build` and `test` targets in `.devkit/contracts/Makefile` only work with contracts within the `.devkit/contracts/` directory and don't include the main `./contracts/` directory where custom contracts are placed.

```makefile
build:
forge clean
forge build

test:
forge test -vvv
```

While there is a `deploy-custom-contracts` target that references custom contracts, the basic `build` and `test` commands don't validate that custom contracts compile correctly or run tests for them.

**Impact**:
- Developers may write custom contracts in `./contracts/` that have compilation errors, but these errors won't be caught until deployment time
- Custom contract tests placed in `./contracts/test/` won't be executed by the standard test command
- No way to run targeted tests for custom contracts during development

### 3. No Fast Contract Compilation Feedback Loop

**Problem**: There's no quick way to verify contract compilation without running the full `devkit avs build` command, which includes many time-consuming steps beyond contract compilation.

**Current Limitations**:
- `devkit avs build` takes significant time due to multiple build steps (binaries, contracts, etc.)
- Calling the build command in the Makefile directly doesn't build contracts outside the `.devkit` folder
- No dedicated contract-only build command for rapid iteration

**Impact**: Slow feedback loops significantly impact development productivity when iterating on contract code.

## Additional Context
These issues became apparent while following the standard DevKit workflow for custom contract development. The current process requires significant manual intervention and deep knowledge of the build system internals, which may deter developers from effectively using DevKit for contract development.

Start by inspecting .devkit/contracts/Makefile, .devkit/scripts/build, and .devkit/contracts/foundry.toml, then compare their targets with the ./contracts/ directory and the devkit avs build flow. Done should include documented dependency setup plus a practical build and test path that covers custom contracts and provides faster compilation feedback.

索引モデルが issue の本文から書いたものです。