When the chat `start_workspace` tool triggers an active-version upgrade
that introduces new required parameters, the build fails with a
parameter validation error. Previously this returned a message telling
the user to update from the UI — a dead end for the model.
This PR lets the model recover inside the chat by:
1. Accepting an optional `parameters` map on `start_workspace` (same
schema as `create_workspace`), forwarded as `RichParameterValues`.
2. Returning structured JSON error responses that preserve validation
details and the workspace's `template_id`, so the model can call
`read_template` to discover what changed.
3. Replacing the UI-only guidance in `exp_chats.go` with
model-actionable retry instructions.
The expected model flow on an active-version parameter failure is now:
```
start_workspace → fails (structured error with template_id + validations)
read_template → discovers new required parameters
start_workspace → retries with parameters map → workspace starts
```
<img width="846" height="511" alt="image"
src="https://github.com/user-attachments/assets/d18b6864-5970-4225-8da0-0f2ab134ccb4"
/>
- Stores a deployment-wide agents template allowlist in `site_configs`
(`agents_template_allowlist`)
- Adds `GET/PUT /api/experimental/chats/config/template-allowlist`
endpoints
- Filters `list_templates`, `read_template`, and `create_workspace` chat
tools by allowlist, if defined (empty=all allowed)
- Add "Templates" admin settings tab in Agents UI ([what it looks
like](https://624de63c6aacee003aa84340-sitjilsyrr.chromatic.com/?path=/story/pages-agentspage-agentsettingspageview--template-allowlist))
> 🤖 This PR was created with the help of Coder Agents, and has been
reviewed by my human. 🧑💻
- Moves `coderd/chatd/`, `coderd/gitsync/`, `enterprise/coderd/chatd/`
under `x/` parent directories to signal instability
- Adds `Experimental:` glue code comments in `coderd/coderd.go`
> 🤖 This PR was created with the help of Coder Agents, and was
reviewed by my human. 🧑💻
Ethan