docs(readme): add "What this is" concept section

[mashware]

What

Adds a "What this is" concept section to the README, between the intro and the existing "How is this different from…" table — positioning the tool by purpose rather than by feature comparison.

It conveys, in short prose:

• Domain Memory is a living glossary of the project's business domain (concepts, rules, entities, the why), framed as semantic memory vs episodic memory (the episodic record already lives in git history).
• Capture is low-friction: the agent records findings as it works, consolidated into entries via /save-knowledge or at PR time — no separate authoring chore.
• The source of truth is plain markdown you own and version in git; the search index is derived and rebuildable.
• One honest boundary line, by category (session history / cross-machine sync / generic assistant memory is not what this is).

Notes

• Every claim is backed by what the project actually does — no invented capabilities (e.g. no "Stop hook").
• Ordering is abstract → concrete: the concept section precedes the existing neighbours table, which is kept intact.
• Docs-only change; no code touched.