docs: add Companion plugins section linking to atlas-for-engram

[Kirilgitlsiiejah]

Adding a small "Companion Plugins" section to the README linking to atlas-for-engram — a community plugin that bridges Obsidian Web Clipper raw clips to engram's project-scoped memory.

Context

atlas-for-engram is a native Claude Code plugin built on top of engram. v0.1.0 was just released. It's an opinionated knowledge management layer (Obsidian + vault + atlas-pool) on top of engram's minimal memory core. We thought engram users searching for ecosystem extensions might find it useful.

Scope

This PR is docs only — no code changes, no engram functionality affected. Just a section in the README pointing at the community plugin with a clear "not officially affiliated" disclaimer.

What changed

• README.md: new "Companion Plugins" section with one row pointing at atlas-for-engram, including a brief description and the disclaimer that it's a community / non-official project. Used a markdown table to match the styling already used elsewhere in the README (Setup, MCP Tools, CLI, Documentation sections).

What it is NOT

• No engram code changes
• No new dependencies introduced
• No commitment to maintain or endorse the linked project beyond the docs link
• Easy to revert if you don't want it

Why community / not in engram core

atlas-for-engram is opinionated about Obsidian/vault/atlas-pool conventions, while engram is intentionally minimal and storage-agnostic. Keeping it as a separate plugin honors that separation while letting engram users discover it.

Happy to adjust wording, tone, section name, or placement if there's a preferred convention for community plugins in the engram README.

Thanks for engram — it's a great foundation to build on.