feat(coderd/templatebuilder): add Compose and BundleTar functions - #26349

…talog

Reusable Go script that reads the Coder registry repo and generates
module.json and .tf.tmpl files for the template builder catalog. Parses
variable declarations from main.tf, frontmatter from README.md, and
pinned versions from git tags.

Usage:
  go run ./scripts/modulegen/ -registry /path/to/registry -output coderd/templatebuilder/modules

Handles escaped quotes and heredoc descriptions, skips variables with
heredoc defaults (cannot be represented in the builder UI), uses numeric
semver sorting for version tags, and exits non-zero on failures.

…dules

…raction

Add ModuleRenderContext and RenderModuleTemplate for rendering module
.tf.tmpl files with registry URL, pinned version, agent resource name,
and variable values. Nil-guards the Variables map to prevent panics.

Extract shared renderTemplate with missingkey=error so missing variable
keys fail loudly instead of producing "<no value>" in rendered HCL.

Add ExtractAgentResourceName which uses a regex to find the coder_agent
resource name from rendered base HCL. Strips comment lines (# and //)
before matching to avoid false positives. Errors if zero or more than
one agent is found.

Add ModuleTemplateFS to expose module template files from the embedded
catalog, with validation that the expected .tf.tmpl file exists.

Compose renders a base template and selected modules into Terraform
source files. It validates modules before rendering: rejects duplicates,
ConflictsWith violations, unknown IDs, and OS incompatibility. For each
module, mergeModuleVariables fills in manifest defaults for non-computed,
non-sensitive variables (with basic JSON type validation), then overlays
caller-supplied values. Required variables without defaults are left
absent so missingkey=error catches the omission at render time.

BundleTar packages the result into a tar archive with reproducible
timestamps (time.Unix(0,0)). Writes main.tf always, modules.tf only
when modules are present.