feat(site/src): nest agent pages under a Coder Agents subsection in AI nav (#26667)

Groups the agent-related AI settings pages under a new **Coder Agents**
parent in the sidebar, with a continuous left rule connecting the
children and an active-segment indicator that lights up the rule where
the current sub-item sits.

The new nav order:

- AI Governance
- AI Gateway keys
- Providers
- Coder Agents
  - Models
  - MCP servers
  - Templates
  - Spend
  - Instructions
  - Lifecycle

All target pages already exist on main (Danielle's recent migrations of
Models, MCP servers, Templates, Instructions, Lifecycle, Spend, and
Coder Agents into AI Settings). This PR only changes the sidebar visual
structure: the children move into an indented group with a `border-l
border-l-border` rule, and the active child paints a
`border-l-content-primary` segment over that rule via `-ml-px` so the
rule and indicator share a column instead of stacking.

<details>
<summary>Design notes</summary>

Concept 1 from the earlier exploration: always-expanded with indents,
the parent is its own page. Chosen because it adds no expand/collapse
state, no "which child is the default" question, and no animation work;
the parent reuses the existing nav-item, and the children sit in a
wrapper `div` with a left rule. The site bundle ships without Tailwind's
preflight, so the wrapper and sub-item borders are paired with
`border-solid` to actually paint, matching the pattern already in
`Sidebar.tsx`.

</details>

---

_This PR was prepared by Coder Agents on behalf of @tracyjohnsonux._

docs/ai-coder/agents/getting-started.md

@@ -195,7 +195,7 @@ deployment. Use this to encode organizational conventions:
 - Required review processes before merging.
 - Any guardrails specific to your environment.
 
-Configure the system prompt from **AI Settings** > **Instructions**
+Configure the system prompt from **AI Settings** > **Coder Agents** > **Instructions**
 or via the API at `PUT /api/experimental/chats/config/system-prompt`.
 See [Platform Controls](./platform-controls/index.md) for details.