init: exclude local .codegraph indexes from git

[Wood-Q]

Summary

Automatically add newly initialized .codegraph/ directories to the local Git exclude file (.git/info/exclude) during codegraph init.

Problem

Today, every time a user runs codegraph init, the generated .codegraph/ directory can appear in git status. In practice, that means users often have to manually add .codegraph/ to .gitignore after initialization.

That is annoying repeated cleanup, and it also creates a product mismatch: .codegraph/ is local generated index state, not source code. Repositories published to GitHub should not need to carry CodeGraph-specific ignore rules just because one developer initialized an index locally. Other users can simply run codegraph init themselves.

Solution

This PR writes the ignore rule to .git/info/exclude instead of the tracked .gitignore.

That gives users the desired behavior locally:

• .codegraph/ no longer appears in git status after init
• project .gitignore remains untouched
• GitHub/released source trees do not gain CodeGraph-specific metadata
• repeated init flows do not duplicate existing exclude entries
• nested project roots in monorepos write the correct relative exclude path

If the project is not inside a Git worktree, or Git metadata cannot be accessed, initialization still succeeds and the exclude step is skipped.

Tests

• Added coverage for adding .codegraph/ to .git/info/exclude
• Added idempotency coverage for existing exclude entries
• Added nested project coverage for monorepo-style paths
• Added non-Git initialization coverage
• Verified with:
  • npm run build
  • npx vitest run __tests__/foundation.test.ts
  • git diff --check