docs: update AI Governance label and add v2.32 requirement (#24708)

## Summary

Replace the "Premium" label with "AI Governance Add-On" and add a
disclaimer that the AI Governance Add-On is required for AI Gateway and
Agent Firewall as of Coder v2.32, across all AI Governance doc pages and
their children.

## Changes

**Label and requirement updates (7 files):**
- `docs/ai-coder/ai-governance.md`: Removed "(Premium)" from title;
updated GA section to state add-on required as of v2.32.
- `docs/ai-coder/ai-gateway/setup.md`: "Premium license" → "AI
Governance Add-On license".
- `docs/ai-coder/ai-gateway/ai-gateway-proxy/setup.md`: "Premium
license" → "AI Governance Add-On".
- `docs/ai-coder/ai-gateway/clients/claude-code.md`: "(Premium feature)"
→ "(AI Governance Add-On)".
- `docs/manifest.json`: `"state": ["premium"]` → `"state": ["ai
governance add-on"]` for 4 nav entries.

**Disclaimer added to all child pages (26 files):**

AI Gateway pages (18):
`index.md`, `setup.md`, `audit.md`, `monitoring.md`, `mcp.md`,
`reference.md`, `ai-gateway-proxy/index.md`,
`ai-gateway-proxy/setup.md`, `clients/index.md`,
`clients/claude-code.md`, `clients/codex.md`, `clients/mux.md`,
`clients/opencode.md`, `clients/factory.md`, `clients/cline.md`,
`clients/kilo-code.md`, `clients/roo-code.md`, `clients/vscode.md`,
`clients/jetbrains.md`, `clients/zed.md`, `clients/copilot.md`

Agent Firewall pages (8):
`index.md`, `version.md`, `landjail.md`, `rules-engine.md`,
`nsjail/index.md`, `nsjail/docker.md`, `nsjail/k8s.md`, `nsjail/ecs.md`

Other: `security.md`

> [!NOTE]
> The `"ai governance add-on"` state value in `manifest.json` is new.
The docs site renderer may need to be updated to support this state
value.

> Generated by Coder Agents

docs/ai-coder/security.md

@@ -1,3 +1,8 @@
+> [!NOTE]
+> Features mentioned on this page, such as AI Gateway and Agent Firewall,
+> require the [AI Governance Add-On](./ai-governance.md). As of Coder v2.32,
+> deployments without the add-on will not be able to access these features.
+
 As the AI landscape is evolving, we are working to ensure Coder remains a secure
 platform for running AI agents just as it is for other cloud development
 environments.