1. Implementation

• [x] 1.1 Add a writeGitignore(absoluteOutDir) helper (or inline the write) in src/plugin.ts that writes <absoluteOutDir>/.gitignore with the standard ignore-all-except-self content
• [x] 1.2 Call the gitignore write at the end of the try block in generateOpenSpecPages(), after all page files are written

2. Tests

• [x] 2.1 Add a test in src/__tests__/plugin.test.ts (or equivalent) that verifies <outDir>/.gitignore exists with the correct content after generateOpenSpecPages() runs
• [x] 2.2 Add a test that verifies re-running generateOpenSpecPages() overwrites a modified .gitignore with the canonical content (idempotency)

3. Documentation

• [x] 3.1 Update README.md to note that the output directory is automatically gitignored and that a manual docs/openspec entry in .gitignore is no longer required