Add Agent Substrate to homepage and core concepts

[sebbycorp]

Summary

Adds Agent Substrate (new in kagent v0.9.7) across the site, plus a small stats refresh.

Homepage (src/app/page.tsx, src/app/globals.css)

• New Agent Substrate card in the Why kagent grid (benefit: fast startup, efficient resources, secure execution)
• New Agent Substrate runtime item in Platform capabilities
• New Agent Substrate floating chip in the hero (.rd-float-l2), hidden on mobile like the other side chips
• Bumped GitHub stars stat from 2,500+ to 3,000+

Docs — Core Concepts (src/app/docs/kagent/concepts/)

• New page agent-substrate/page.mdx: what it is, why it matters, core concept table (Actor, WorkerPool, ActorTemplate, Snapshot, Session), how snapshot-and-restore works, control/data-plane architecture, and how to use it via AgentHarness runtime: substrate
• QuickLink added to the concepts index; Agent Memory reordered after it
• Regenerated navigation.json / sitemap.xml

Verification

• check-links: 454 links, no broken links
• Homepage and /docs/kagent/concepts/agent-substrate render locally (HTTP 200)

Sources: in-repo API reference (AgentHarnessSubstrateSpec) and https://learn.agentsubstrate.dev/

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds “Agent Substrate” as a first-class concept across the marketing homepage and the kagent Core Concepts docs, and refreshes a small community stat.

Changes:

• Adds an Agent Substrate hero chip + homepage capability/value copy, and updates the GitHub stars stat to 3,000+.
• Introduces new Core Concepts doc page /docs/kagent/concepts/agent-substrate and links it from the concepts index + navigation.
• Regenerates site navigation artifacts (notably navigation.json and sitemap.xml).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/config/navigation.json	Adds Agent Substrate to the Core Concepts nav list.
src/app/page.tsx	Adds Agent Substrate marketing content (chip + cards) and updates GitHub stars stat.
src/app/globals.css	Adds positioning for the new floating chip and hides it on mobile.
src/app/docs/kagent/concepts/page.mdx	Adds a QuickLink to the new Agent Substrate concept page.
src/app/docs/kagent/concepts/agent-substrate/page.mdx	New documentation page describing Agent Substrate concepts, architecture, and usage.
src/app/docs/kagent/concepts/agent-memory/page.mdx	Adjusts pageOrder to accommodate the new concept page.
public/sitemap.xml	Updates lastmod dates, but currently has sitemap URL issues that need correction.

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