docs: audit and update litegraph documentation (#8588)

## Summary

Audits and updates litegraph documentation files that came from the old
litegraph.js repo merged via git subtree (per #8341 discussion).

## Changes

- **Delete** `CONTRIBUTING.md` - completely outdated, referenced
original jagenjo/litegraph.js from 2020
- **Fix** `API.md` - remove Subgraph from "Removed public interfaces"
(actively used in 25+ files)
- **Update** `README.md` - replace standalone releasing instructions
with note about embedded subtree workflow
- **Fix** `AGENTS.md` and `__fixtures__/README.md` - correct import path
typos (`./fixtures/` → `./__fixtures__/`)

Fixes #8347

cc @DrJKL

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Documentation

* Updated release information to clarify that the library is embedded
via git subtree and managed through the parent repository's release
process
* Removed contribution guidelines documentation file
* Updated API documentation by removing Subgraph from the list of
removed public interfaces
* Updated documentation examples to align with current project structure

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8588-docs-audit-and-update-litegraph-documentation-2fc6d73d365081fca335c78ce8493d3e)
by [Unito](https://www.unito.io)

src/lib/litegraph/README.md

@@ -144,18 +144,7 @@ Litegraph has no runtime dependencies. The build tooling has been tested on Node
 
 ## Releasing
 
-Use GitHub actions to release normal versions.
-
-1. Run the `Release a New Version` action, selecting the version increment type
-1. Merge the resolution PR
-1. A GitHub release is automatically published on merge
-
-### Pre-release
-
-The action directly translates `Version increment type` to the pnpm version command. `Pre-release ID (suffix)` is the option for the `--preid` argument.
-
-e.g. Use `prerelease` increment type to automatically bump the patch version and create a pre-release version. Subsequent runs of prerelease will update the prerelease version only.
-Use `patch` when ready to remove the pre-release suffix.
+This library is embedded via git subtree in ComfyUI_frontend. Releases are managed through the parent repository's release process.
 
 ## Contributors