fix(installer): preserve trailing TOML array tables

[KtzeAbyss]

Summary

• Preserve sibling TOML tables and arrays of tables when Codex install, upgrade, or uninstall rewrites the CodeGraph MCP block.
• Detect only actual TOML table headers, so bracket-like lines inside multiline strings or multiline values are not treated as boundaries.
• Support valid whitespace, dotted keys, quoted keys, comments, and CRLF while preserving sibling configuration verbatim.

Testing

• npx vitest run __tests__/installer-targets.test.ts (162 passed)
• npm run build
• git diff --check upstream/main...HEAD

Closes #1351