docs(readme): link the essay, the compare page, the when-to-use page and the deck

Three small additions, no prose removed:

- a line of three links under the demo GIF: Why it exists, How it
  compares, When to use it;
- two pointer sentences closing "What we mean by ontology", linking the
  compare page and the when-to-use page;
- the deck PDF beside "Read why" in the "You own it" quote block.

All targets are absolute objectstack.ai URLs, like the README's other
site links.

Claude-Session: https://claude.ai/code/session_011hRnra93sK5Q2gTYYTbdJR
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

README.md

@@ -21,6 +21,8 @@
   <sub><b>1</b> · One definition, and the app runs &nbsp;·&nbsp; <b>2</b> · An agent creates a record over MCP &nbsp;·&nbsp; <b>3</b> · A read-only identity is refused</sub>
   <br>
   <a href="https://youtu.be/CX_FlOoOtr0"><b>▶&nbsp; Watch: ObjectStack in 90 Seconds</b></a>
+  <br>
+  <sub><a href="https://objectstack.ai/blog/the-ontology-is-the-software">Why it exists</a> &nbsp;·&nbsp; <a href="https://objectstack.ai/docs/concepts/compare">How it compares</a> &nbsp;·&nbsp; <a href="https://objectstack.ai/docs/concepts/when-to-use">When to use it</a></sub>
 </p>
 
 You build & ask with Claude Code or any coding agent: the agent writes the
@@ -88,6 +90,11 @@ runtime, as hooks, action bodies, CEL, and constrained JSX. The full account is
 [Business Ontology](https://objectstack.ai/docs/concepts/ontology); the short
 form is the [glossary entry](https://objectstack.ai/docs/getting-started/glossary#business-ontology).
 
+[How it compares](https://objectstack.ai/docs/concepts/compare) sets ObjectStack
+beside knowledge ontologies, semantic layers, operational ontologies, metadata
+platforms, and AI code generation. [When to use it](https://objectstack.ai/docs/concepts/when-to-use)
+says where it fits today and where it does not.
+
 ## The runtime runs it
 
 Point an agent at an empty repo and you get a one-off codebase: every screen
@@ -207,7 +214,7 @@ graph held inside a vendor's system.
 > The ontology is the software. Your objects, relations, actions, permissions,
 > flows, and agent and tool definitions are your business ontology — and the
 > definition layer of the AI era should be an open protocol you own.
-> [Read why](https://objectstack.ai/blog/the-ontology-is-the-software).
+> [Read why](https://objectstack.ai/blog/the-ontology-is-the-software) · [the talk as a deck (PDF)](https://objectstack.ai/decks/the-ontology-is-the-software.pdf).
 
 ## Ship it