fix(filebrowser): require agent_name when subdomain is false (#877)

## Description

Fixes
[REG-4](https://linear.app/codercom/issue/REG-4/filebrowser-appends-workspace-path-twice-in-url):
the `filebrowser` module opens to a non-existent URL with the workspace
path appended a second time when `subdomain = false` and `agent_name` is
not provided, e.g.:

```
https://<coder-host>/@<owner>/<workspace>/apps/filebrowser/files/@<owner>/<workspace>.<agent>/apps/filebrowser/
```

### Root cause

Coder's frontend always builds path-based app URLs as
`/@<owner>/<workspace>.<agent>/apps/<slug>/` (it always includes
`.<agent_name>`, even for single-agent templates):

https://github.com/coder/coder/blob/main/site/src/modules/apps/apps.ts

```ts
return `${path}/@${workspace.owner_name}/${workspace.name}.${agent.name}/apps/${app.slug}/`;
```

The filebrowser module, however, only includes the agent segment in
`local.server_base_path` (which becomes filebrowser's `--baseURL`) when
the user explicitly passes `agent_name`. The variable description and
the README both said `agent_name` was "only required if the template
uses multiple agents", which is incorrect.

When the URLs disagree, filebrowser's reverse-proxy `stripPrefix` cannot
strip the prefix, the path falls through filebrowser's `/:catchAll(.*)*`
Vue route, and the router redirects to `/files/${catchAll}` — producing
the duplicated path the user reported.

### Fix

- Add a `lifecycle.precondition` on `coder_script.filebrowser` that
fails `terraform apply` with a clear, actionable error when `subdomain =
false` and `agent_name == null`.
- Update the `agent_name` variable description to state it is required
whenever `subdomain` is `false`.
- Update the `README.md` example for the path-based config to call out
the requirement explicitly.
- Bump the module version from `1.1.4` → `1.1.5`.
- Add a TS test covering the new precondition.

This avoids the silent misconfiguration that produces the duplicated
URL, without breaking anyone whose existing template already sets
`agent_name` (or uses `subdomain = true`).

## Type of Change

- [ ] New module
- [ ] New template
- [x] Bug fix
- [ ] Feature/enhancement
- [ ] Documentation
- [ ] Other

## Module Information

**Path:** `registry/coder/modules/filebrowser`  
**New version:** `v1.1.5`  
**Breaking change:** [ ] Yes [x] No

## Testing & Validation

- [x] `bun test main.test.ts` — 8 pass, 0 fail (includes new
precondition test)
- [x] `terraform fmt -recursive`
- [x] `terraform validate`
- [x] `bun x prettier --check`
- [x] Manually verified the precondition fires with a minimal repro and
passes when `agent_name` is supplied or `subdomain = true`.

## Related Issues

- Linear:
[REG-4](https://linear.app/codercom/issue/REG-4/filebrowser-appends-workspace-path-twice-in-url)

---

Created on behalf of @matifali.

Generated with Blink.

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
Co-authored-by: DevCats <christofer@coder.com>

registry/coder/modules/filebrowser/README.md

@@ -14,7 +14,7 @@ A file browser for your workspace.
 module "filebrowser" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/filebrowser/coder"
-  version  = "1.1.4"
+  version  = "1.1.5"
   agent_id = coder_agent.main.id
 }
 ```
@@ -29,7 +29,7 @@ module "filebrowser" {
 module "filebrowser" {
   count    = data.coder_workspace.me.start_count
   source   = "registry.coder.com/coder/filebrowser/coder"
-  version  = "1.1.4"
+  version  = "1.1.5"
   agent_id = coder_agent.main.id
   folder   = "/home/coder/project"
 }
@@ -41,7 +41,7 @@ module "filebrowser" {
 module "filebrowser" {
   count         = data.coder_workspace.me.start_count
   source        = "registry.coder.com/coder/filebrowser/coder"
-  version       = "1.1.4"
+  version       = "1.1.5"
   agent_id      = coder_agent.main.id
   database_path = ".config/filebrowser.db"
 }
@@ -49,11 +49,13 @@ module "filebrowser" {
 
 ### Serve from the same domain (no subdomain)
 
+When `subdomain = false`, you must also set `agent_name` to the name of your `coder_agent` resource. Coder serves path-based apps at `/@<owner>/<workspace>.<agent>/apps/<slug>/`, so the agent name is required to build a base URL that matches the URL the user is actually browsing. If `agent_name` is omitted in this mode, `terraform apply` will fail with an explanatory error.
+
 ```tf
 module "filebrowser" {
   count      = data.coder_workspace.me.start_count
   source     = "registry.coder.com/coder/filebrowser/coder"
-  version    = "1.1.4"
+  version    = "1.1.5"
   agent_id   = coder_agent.main.id
   agent_name = "main"
   subdomain  = false

registry/coder/modules/filebrowser/main.test.ts

@@ -102,4 +102,19 @@ describe("filebrowser", async () => {
 
     testBaseLine(output);
   }, 15000);
+
+  it("fails when subdomain=false and agent_name is not provided", async () => {
+    let caught: Error | undefined;
+    try {
+      await runTerraformApply(import.meta.dir, {
+        agent_id: "foo",
+        subdomain: false,
+      });
+    } catch (e) {
+      caught = e as Error;
+    }
+    expect(caught).toBeDefined();
+    expect(caught!.message).toContain("agent_name");
+    expect(caught!.message).toContain("subdomain");
+  }, 15000);
 });

registry/coder/modules/filebrowser/main.tf

@@ -20,7 +20,7 @@ data "coder_workspace_owner" "me" {}
 
 variable "agent_name" {
   type        = string
-  description = "The name of the coder_agent resource. (Only required if subdomain is false and the template uses multiple agents.)"
+  description = "The name of the coder_agent resource. Required when `subdomain` is `false` so the path-based base URL matches the URL Coder serves."
   default     = null
 }
 
@@ -102,6 +102,13 @@ resource "coder_script" "filebrowser" {
     SERVER_BASE_PATH : local.server_base_path
   })
   run_on_start = true
+
+  lifecycle {
+    precondition {
+      condition     = var.subdomain || var.agent_name != null
+      error_message = "`agent_name` is required when `subdomain` is `false`. Coder always builds path-based app URLs as `/@<owner>/<workspace>.<agent>/apps/<slug>/`, so the filebrowser base URL must include the agent name to match. Set `agent_name = \"<your coder_agent name>\"` (e.g. `\"main\"`)."
+    }
+  }
 }
 
 resource "coder_app" "filebrowser" {