docs: refresh architecture and extensibility pages

[drew]

Summary

Tighten the README, architecture, and extensibility docs. The README shows how to install the Rust SDK, the architecture page leads with policy enforcement and uses consistent component names, and the extensibility overview gets a simpler diagram that shows which layers each extension point extends.

Related Issue

No issue required: documentation-only changes.

Changes

README

• Replace the Rust SDK table description with the cargo add command and link the row to the Rust SDK guide.

Architecture (docs/about/architecture.mdx)

• Lead with how OpenShell enforces policy (kernel-level enforcement and formally verified policy changes), reusing the README's "How it works" text.
• Merge the gateway and workload creation sections into one paragraph under the intro.
• Consistently bold and name the sandbox and supervisor components instead of mixing openshell-sandbox and "OpenShell Sandbox".
• Rename "The protected channel" to "Mediated Channel", shorten the network request walkthrough, and remove the "Starting an agent safely" section.
• Refresh the architecture diagrams: the system architecture diagram adds a policy lifecycle strip (agent proposes, gateway prover checks, you approve, policy reloads), the sandbox enforcement diagram shows policy.local propose/reload in place of the launch gate, and the remaining diagrams get matching style and alt-text updates.

Extensibility (docs/extensibility/)

• Redraw the extension points diagram as two left-to-right rows. The data plane row shows the isolation backend and supervisor middleware; the control plane row shows gateway interceptors and drivers.
• Add a short list to the overview intro that maps the control plane, data plane, and infrastructure layers to their extension points.
• Move the authentication section after "Building Extensions" and rename it "Authenticating Extensions". Update links from the gateway interceptors and supervisor middleware pages to the new anchor.

Testing

• mise run pre-commit passes
• Diagrams render and are valid XML
• Unit tests added/updated (not applicable to documentation changes)
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; published docs only)

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