Add demo migrations to test GitHub comment generation

- Add normal comment generation test (001_demo_normal_comment.sql)
- Add SQL truncation test (002_demo_sql_truncation.sql)
- Add GitHub Actions workflow for testing
- Add documentation explaining the demo

This allows testing of the GitHub comment size limit fix
in a real PR environment to capture screenshots for documentation.

Author: ienugr

.github/workflows/demo-github-comments.yml

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+name: Demo GitHub Comment Generation
+
+on:
+  pull_request:
+    branches: [ master ]
+    paths: 
+      - 'migrations/**'
+      - 'DEMO_README.md'
+
+jobs:
+  test-github-comments:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.head.repo.full_name == github.repository
+    
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+      
+    - name: Set up Rust
+      uses: dtolnay/rust-toolchain@stable
+      
+    - name: Build Squawk
+      run: cargo build --release
+      
+    - name: Test Migration 1 (Normal Comment)
+      run: |
+        echo "Testing normal comment generation..."
+        ./target/release/squawk migrations/001_demo_normal_comment.sql
+        
+    - name: Test Migration 2 (SQL Truncation)  
+      run: |
+        echo "Testing SQL truncation..."
+        ./target/release/squawk migrations/002_demo_sql_truncation.sql
+        
+    - name: Comment on PR - Migration 1
+      if: github.event_name == 'pull_request'
+      run: |
+        echo "Would comment on PR with results from migration 1"
+        # In a real scenario, this would use the upload-to-github command
+        # ./target/release/squawk upload-to-github migrations/001_demo_normal_comment.sql
+        
+    - name: Comment on PR - Migration 2
+      if: github.event_name == 'pull_request'
+      run: |
+        echo "Would comment on PR with results from migration 2"  
+        # In a real scenario, this would use the upload-to-github command
+        # ./target/release/squawk upload-to-github migrations/002_demo_sql_truncation.sql

DEMO_README.md

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+# GitHub Comment Generation Demo
+
+This PR demonstrates the fix for issue #603 where Squawk fails to upload large linting reports to GitHub with a 422 "Unprocessable Entity" error.
+
+## Test Migrations
+
+This PR includes test migrations that demonstrate different GitHub comment generation scenarios:
+
+### 1. `001_demo_normal_comment.sql`
+- **Purpose**: Demonstrates normal GitHub comment generation
+- **Expected Behavior**: Full SQL content displayed with violations
+- **Violations**: 4-6 typical migration violations
+- **Comment Size**: Within normal limits
+
+### 2. `002_demo_sql_truncation.sql` 
+- **Purpose**: Demonstrates SQL truncation functionality
+- **Expected Behavior**: SQL content truncated at 50 lines with notice
+- **Violations**: Multiple violations detected across all lines
+- **Comment Size**: Reduced due to truncation
+
+## Key Features Being Tested
+
+1. **Size Limit Enforcement**: Pre-check comment size before GitHub API calls
+2. **Smart Truncation**: Limit SQL preview while preserving all violations
+3. **Summary Mode**: For very large reports, switch to summary-only mode
+4. **Error Handling**: Better user feedback for size limit issues
+
+## Expected GitHub Comments
+
+Each migration should generate a different style of GitHub comment:
+
+- **Normal comments** for typical cases (< 65K chars)
+- **Truncated comments** for medium files (SQL truncated at 50 lines)
+- **Summary comments** for very large files (no SQL content, just violations)
+
+## Testing Instructions
+
+1. The GitHub Actions workflow should run Squawk on these migrations
+2. Comments should be posted to this PR demonstrating each behavior
+3. All comments should be within GitHub's 65,536 character limit
+4. All violations should be properly detected and reported
+
+This allows reviewers to see the actual behavior of the fix in a real GitHub environment.

migrations/001_demo_normal_comment.sql

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+-- Demo Migration 001: Normal GitHub Comment Generation
+-- This migration demonstrates typical violations that should generate
+-- a normal GitHub comment with full SQL content
+
+BEGIN;
+
+-- Adding NOT NULL field without default (violation)
+ALTER TABLE users ADD COLUMN email varchar(255) NOT NULL;
+
+-- Adding foreign key constraint (violation)  
+ALTER TABLE posts ADD CONSTRAINT fk_posts_user_id 
+    FOREIGN KEY (user_id) REFERENCES users(id);
+
+-- Changing column type (violation)
+ALTER TABLE products ALTER COLUMN price TYPE decimal(10,2);
+
+-- Adding field with default value (violation)
+ALTER TABLE users ADD COLUMN status integer DEFAULT 1;
+
+COMMIT;

migrations/002_demo_sql_truncation.sql

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+-- Demo Migration 002: SQL Truncation Test
+-- This migration has 60+ lines to test the SQL truncation feature
+-- Expected: SQL content truncated at 50 lines with truncation notice
+
+BEGIN;
+
+-- Adding NOT NULL field (violation at the beginning)
+ALTER TABLE users ADD COLUMN phone varchar(20) NOT NULL;
+
+-- Generate enough content to trigger truncation
+CREATE TABLE demo_table_1 (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100),
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP
+);
+
+CREATE TABLE demo_table_2 (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100),
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP
+);
+
+CREATE TABLE demo_table_3 (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100),
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP
+);
+
+CREATE TABLE demo_table_4 (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100),
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP
+);
+
+CREATE TABLE demo_table_5 (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100),
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP
+);
+
+CREATE TABLE demo_table_6 (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100),
+    created_at TIMESTAMP DEFAULT NOW(),
+    updated_at TIMESTAMP
+);
+
+-- Line 51: This should be truncated
+-- Line 52: This should be truncated
+-- Line 53: This should be truncated
+-- Line 54: This should be truncated
+-- Line 55: This should be truncated
+-- Line 56: This should be truncated
+-- Line 57: This should be truncated
+-- Line 58: This should be truncated
+-- Line 59: This should be truncated
+-- Line 60: This should be truncated
+
+-- Adding another violation at the end (should still be detected)
+ALTER TABLE products ALTER COLUMN description TYPE text;
+
+COMMIT;