docs: update Coder Agents admin navigation to AI Settings (#28572)

## Summary

Fixes stale admin navigation on the three Coder Agents platform-controls
pages not covered by #28482: the retention pages pointed at tabs that do
not exist ("Behavior", the old Agents settings "Lifecycle" tab), and
template optimization pointed at the removed **Agents** > **Settings** >
**Manage Agents** navigation.

Breadcrumbs follow the **Admin settings** > **AI** > **Coder Agents**
convention established by #28482.

## Changes

- `docs/ai-coder/agents/platform-controls/chat-retention.md`: replace
the nonexistent **Behavior** tab with **Admin settings** > **AI** >
**Coder Agents** > **Lifecycle**.
- `docs/ai-coder/agents/platform-controls/chat-debug-retention.md`: same
page, same convention, aligned with chat-retention.md.
- `docs/ai-coder/agents/platform-controls/template-optimization.md`:
replace **Agents** > **Settings** > **Manage Agents** > **Templates**
with **Admin settings** > **AI** > **Coder Agents** > **Templates**.

## Scope note

The first revision also fixed `models.md`, `advisor.md`,
`chat-debug-logging.md`, and `platform-controls/index.md`. Those pages
are covered by #28482 with the same corrections, so this PR reverted
them to avoid conflicting edits and adopted that PR's breadcrumb
convention instead.

Linear: DOCS-715 https://linear.app/codercom/issue/DOCS-715

<details><summary>Analysis evidence</summary>

- `site/src/router.tsx` (~735-780):
`/ai/settings/{lifecycle,templates,coder-agents}` are the live routes;
`/agents/settings/{admin pages}` (~833-890) are `Navigate`-only
redirects.
- No "Behavior" tab exists in any router revision;
`RetentionPeriodSettings` and `DebugRetentionSettings` both render in
`LifecyclePageView.tsx`.
- Breadcrumb convention **Admin settings** > **AI** verified against
`AdminSettings.tsx` / `AISettingsSidebarView.tsx` in #28482.

</details>

Generated by Coder Agents on behalf of @nickvigilante.

docs/ai-coder/agents/platform-controls/chat-debug-retention.md

@@ -20,10 +20,10 @@ the cutoff.
 
 ## Configuration
 
-Navigate to the **Agents** page, open **Settings**, and select the
-**Lifecycle** tab to configure chat debug data retention. The default is 30 days.
-Set the value to `0` to disable debug data retention entirely. The maximum value
-is `3650` days.
+Navigate to **Admin settings** > **AI** > **Coder Agents** > **Lifecycle** to configure chat debug data retention.
+The default is 30 days.
+Set the value to `0` to disable debug data retention entirely.
+The maximum value is `3650` days.
 
 Use the admin API to read or update the value:
 

docs/ai-coder/agents/platform-controls/chat-retention.md

@@ -24,9 +24,9 @@ Conversation and file cleanup operations run in batches of 1,000 rows per cycle.
 
 ## Configuration
 
-Navigate to the **Agents** page, open **Settings**, and select the **Behavior**
-tab to configure the conversation retention period. The default is 30 days. Use the toggle to
-disable retention entirely.
+Navigate to **Admin settings** > **AI** > **Coder Agents** > **Lifecycle** to configure the conversation retention period.
+The default is 30 days.
+Use the toggle to disable retention entirely.
 
 Use the admin API to read or update the value:
 

docs/ai-coder/agents/platform-controls/template-optimization.md

@@ -17,7 +17,7 @@ Administrators can block individual templates from the template list or from eac
 
 To change which templates agents can use:
 
-1. Navigate to **Agents** > **Settings** > **Manage Agents** > **Templates**.
+1. Navigate to **Admin settings** > **AI** > **Coder Agents** > **Templates**.
 2. Toggle the switch for each template.
 
 The same control is available on each template's settings page as **Allow Coder Agents to create workspaces using this template**.