docs: close Coder Agents coverage gaps and align nav references (#24971)

Closes coverage gaps in `docs/ai-coder/agents/` and aligns nav
references with the current UI (post #24574 Behavior split, post #24644
Insights removal).

**Content fixes:**

- Replace site-wide `coder users edit-roles` flow with org-scoped
`agents-access` role (per migration `000475`). CLI examples now preserve
existing org roles since `edit-roles` overwrites the full set.
- Correct computer-use claim: supports Anthropic *and* OpenAI providers,
configured under the Virtual desktop experiment.
- New `platform-controls/experiments.md` covering Virtual desktop,
Advisor, and Chat debug logging (each as: what, how to enable, API).
Includes the Debug tab in the chat right panel.
- Trim `models.md` "Model overrides" to essentials: two layers (admin
subagent, user personal), contexts table, resolution order, API pointer.
- Remove retired `platform-controls/pr-insights.md` (page + manifest +
cross-links).

**Nav cleanup:**

- Admin-only tabs use the full `Agents > Settings > Manage Agents >
<Tab>` path; user-side tabs keep `Agents > Settings > <Tab>`.
- Replace stale "Behavior" references with Instructions / Lifecycle /
Experiments to match the current sidebar.
- Replace references to the removed top-bar Admin dialog with the
Settings sidebar.

<details>
<summary>Decision log</summary>

- Experimental features were originally drafted as a standalone Advisor
page plus inline sections in `platform-controls/index.md`. Consolidated
into one `experiments.md` since no individual feature warrants a full
page yet and parallel short sections are easier to scan.
- Reviewer feedback on early drafts: drop the inline experiments list
from `index.md` (avoid drift), drop the "users created before this role
was introduced" note (handled transparently by migration `000475`),
specify the full nav path for per-model pricing, link the
`type=computer_use` row in `architecture.md` to the Experiments page.
- CLI bulk-grant script previously called `edit-roles <user>
agents-access`. That replaces the user's full org role set, so the
script would silently strip `organization-admin`,
`organization-template-admin`, etc. Rewrote to read each user's current
roles, append `agents-access`, dedupe, and write the union back.

</details>

PR generated with Coder Agents.

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

@@ -20,7 +20,7 @@ template allowlist.
 
 To configure the allowlist:
 
-1. Navigate to **Agents** > **Settings** > **Templates**.
+1. Navigate to **Agents** > **Settings** > **Manage Agents** > **Templates**.
 2. Select the templates you want agents to be able to use.
 3. Click **Save**.