Implement `GET /api/v2/templatebuilder/modules`, which returns the
filtered list of modules available for a given base template. Reads from
the bundled catalog via `LoadModules()` and applies OS-compatibility
filtering based on the `base` query param.
Computed variables (e.g. `agent_id`) are excluded from the API response
at the `ToSDK()` conversion boundary since they are wired automatically
by the builder. The `Computed` field is removed from the SDK type. Adds
`CompatibleWithOS()` to `ModuleManifest` for OS filtering.
Returns 400 for unknown base IDs and 404 when the template builder is
disabled.
Depends on #26116
> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.
Scaffolds the `coderd/templatebuilder` package for the guided template
builder ([DEVEX-272](https://linear.app/codercom/issue/DEVEX-272),
[RFC](https://www.notion.so/coderhq/RFC-Guided-Template-Creation-Workflow-342d579be59280dfbf8eea2e5006dbda)).
Adds the module catalog types and `go:embed` wiring that the template
builder endpoints will use:
- `codersdk.TemplateBuilderModule`, `TemplateBuilderModuleVariable`, and
related types matching the RFC schema
- Internal `ModuleManifest` type with `go:embed` wiring to bundle
`module.json` files from `coderd/templatebuilder/modules/`
- `LoadModules()` with defensive copy, unexported
`parseModulesFromFS(fs.FS)` for test isolation, `ToSDK()` conversion
- Real `code-server` module manifest as the first catalog entry
- Strict validation: ID uniqueness, version non-empty, variable
type/name validation, `DisallowUnknownFields`, and requiring
`module.json` in every module directory
- Tests via internal `catalog_internal_test.go` (for
`parseModulesFromFS` with `fstest.MapFS` fixtures) and external
`catalog_test.go` (for `LoadModules` and `ToSDK`), covering multi-module
parsing, all variable types, validation errors, nil-slice normalization,
and full SDK field assertions
> [!NOTE]
> Generated with [Coder Agents](https://coder.com/agents) by
@jeremyruppel
---------
Co-authored-by: McKayla はな <mckayla@hey.com>
Jeremy Ruppel