feat: zero-config indexing driven by .gitignore (#283) (#285)

Remove .codegraph/config.json and the entire config surface. CodeGraph now
indexes every file whose extension maps to a supported language and respects
.gitignore everywhere — git repos via git itself, non-git projects via the
`ignore` library (root + nested .gitignore files, the same way git does).

- Remove CodeGraphConfig/DEFAULT_CONFIG, src/config.ts, and the public config
  API (the `config` option on init, getConfig/updateConfig/getConfigPath).
- Derive the source-file allowlist from EXTENSION_MAP (isSourceFile); maxFileSize
  is now a constant. Drop the .codegraphignore marker.
- Behavior change: committed, non-gitignored dirs (vendor/, a committed dist/)
  are now indexed — .gitignore is the single source of truth.

Earlier inert fields (languages, frameworks, extractDocstrings, trackCallSites,
customPatterns) and their dead helpers are removed as part of this.

Resolves #283.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>