docs: document per-template agents_allowed (#27518)

Relates to CODAGT-713

Depends on #27517

This updates the Coder Agents platform controls documentation for the
per-template `agents_allowed` model. It replaces the deployment-wide
allowlist instructions with the **Agents allowed** controls in AI
Settings and template settings, documents that templates allow agents by
default, and explains that disabled templates are excluded from
`list_templates`, `read_template`, and `create_workspace`.

This is the final PR in the stack and aligns the template routing and
optimization guidance with the database, API, frontend, cleanup, and CLI
changes in the preceding PRs.

docs/ai-coder/agents/tools/index.md

@@ -44,8 +44,7 @@ code), it provisions a workspace using three tools:
 | `read_template`    | Read a template's parameters and presets                  |
 | `create_workspace` | Create the workspace from a chosen template               |
 
-Administrators can restrict which templates these tools can see with the
-[template allowlist](../platform-controls/template-optimization.md#restrict-available-templates).
+Administrators can restrict which templates these tools can use with the switches on the [Templates page](../platform-controls/template-optimization.md#restrict-available-templates).
 
 ### list_templates
 
@@ -214,4 +213,4 @@ Guardrails:
   recommended.
 - The tool is idempotent: if the chat already has a workspace building or
   running, that workspace is returned instead of creating a duplicate.
-- Templates outside the administrator's allowlist are rejected.
+- Templates that do not allow Coder Agents are rejected.