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/agents/models.md

@@ -257,12 +257,12 @@ until you add a new personal key.
 ## Using an LLM proxy
 
 Organizations that route LLM traffic through a centralized proxy — such as
-Coder's AI Bridge or third parties like LiteLLM — can point any provider's **Base URL** at their proxy endpoint.
+Coder's AI Gateway or third parties like LiteLLM — can point any provider's **Base URL** at their proxy endpoint.
 
-For example, to route all OpenAI traffic through Coder's AI Bridge:
+For example, to route all OpenAI traffic through Coder's AI Gateway:
 
 1. Add or edit the **OpenAI** provider.
-1. Set the **Base URL** to your AI Bridge endpoint
+1. Set the **Base URL** to your AI Gateway endpoint
    (e.g., `https://example.coder.com/api/v2/aibridge/openai/v1`).
 1. Enter the API key your proxy expects.