docs: rename AI Bridge to AI Gateway and Agent Boundaries to Agent Firewall (#24094)

*Disclaimer: implemented by a Coder Agent using Claude Opus 4.6*

## Summary

Renames product references across documentation:

| Old Name | New Name |
|----------|----------|
| AI Bridge | AI Gateway |
| AI Bridge Proxy | AI Gateway Proxy |
| Agent Boundaries | Agent Firewall |

## What changed

- Prose text, headings, titles, and descriptions updated across all docs
- Directories renamed:
  - `docs/ai-coder/ai-bridge/` → `docs/ai-coder/ai-gateway/`
- `docs/ai-coder/ai-bridge/ai-bridge-proxy/` →
`docs/ai-coder/ai-gateway/ai-gateway-proxy/`
  - `docs/ai-coder/agent-boundaries/` → `docs/ai-coder/agent-firewall/`
- All internal markdown links updated to new paths
- `manifest.json` route paths updated
- Rename notice added to AI Gateway and Agent Firewall entrypoint pages

## Companion PR

URL redirects (old paths → new paths):
[coder/coder.com#700](https://github.com/coder/coder.com/pull/700)

## What is intentionally NOT changed

- **Env vars**: `CODER_AIBRIDGE_*`
- **CLI flags**: `--aibridge-*`
- **API paths**: `/api/v2/aibridge/*`
- **Config keys**: `aibridge:` YAML blocks
- **Terraform variables**: `enable_aibridge`, `boundary_version`,
`use_boundary_directly`
- **Process names**: `aibridged`, `aibridgeproxyd`
- **Prometheus metrics**: `coder_aibridged_*`, `coder_aibridgeproxyd_*`
- **SDK types**: `codersdk.AIBridge*`
- **GitHub URLs**: `github.com/coder/aibridge`
- **Image paths**: `images/aibridge/`
- **Auto-generated reference docs**: `docs/reference/cli/aibridge*.md`,
`docs/reference/api/aibridge.md`, `docs/reference/api/schemas.md`
- **Frontend code**: `site/src/` references (separate PR)

Code-level renames (env vars, configs, frontend) are planned for a
follow-up PR.

docs/ai-coder/index.md

@@ -31,18 +31,18 @@ terminal-based agent such as Claude Code or Codex's Open Source CLI.
 [Learn more about Coder Tasks](./tasks.md) for best practices and how to get
 started.
 
-## Secure Your Workflows with Agent Boundaries
+## Secure Your Workflows with Agent Firewall
 
 AI agents can be powerful teammates, but must be treated as untrusted and
 unpredictable interns as opposed to tools. Without the right controls, they can
 go rogue.
 
-[Agent Boundaries](./agent-boundaries/index.md) is a new tool that offers
+[Agent Firewall](./agent-firewall/index.md) is a new tool that offers
 process-level safeguards that detect and prevent destructive actions. Unlike
 traditional mitigation methods like firewalls, service meshes, and RBAC systems,
-Agent Boundaries is an agent-aware, centralized control point that can either be
+Agent Firewall is an agent-aware, centralized control point that can either be
 embedded in the same secure Coder Workspaces that enterprises already trust, or
 used through an open source CLI.
 
 To learn more about features, implementation details, and how to get started,
-check out the [Agent Boundaries documentation](./agent-boundaries/index.md).
+check out the [Agent Firewall documentation](./agent-firewall/index.md).