fix(template): children were red because ./... cannot see the entrypoint

Closes #1. Closes #2.

A repo generated from this template failed `task check` on errcheck from its
first commit, while the template's own `task check` was green. The reason is
not in the code:

    Go ignores directories whose names begin with "_" or "." when expanding
    package patterns like ./...

The entrypoint lives in cmd/__PROJECT_NAME__. So `go build ./...`,
`go vet ./...`, `go test ./...` and `golangci-lint run ./...` ALL silently
skip the one package every generated repo is built around:

    $ go list ./...
    __MODULE_PATH__/internal/agent
    __MODULE_PATH__/pkg/litellm        <- no cmd package

After substitution the directory has a real name, becomes visible, and the
defect appears. Template green, children red, by construction -- and invisible
to anyone reading either repo's CI.

Fixes the one live defect: `defer shutdown(ctx)` dropped an error, and
SetupTelemetry returns func(context.Context) error.

Two of the three defects these issues describe were already fixed and neither
issue knew: go.sum has been committed for some time, and resp.Body.Close is
already wrapped. Verified before touching anything rather than trusted from
the issue text.

Adds scripts/tests/generated-child_test.sh, wired into `task check`: it
substitutes the tracked tree into a throwaway child and gates go list / build
/ vet / golangci-lint on the RESULT. Without it the next entrypoint defect
rots exactly the same way.

Not done by adding ./cmd/__PROJECT_NAME__ to the lint command, which does work
-- naming a path directly bypasses the pattern exclusion -- because it reports
a gofumpt violation on the import block that does not exist in any child. The
formatter's import grouping depends on the module path and the template's is a
placeholder, so that route turns the gate red over an artifact of being a
template.

The harness reads tracked files from the working tree rather than
`git archive HEAD`: it runs inside `task check`, so it must gate what is about
to be committed. Archiving HEAD reported a stale pass against an unstaged fix
on first use here.

Proven to fail when it should: red on the unfixed entrypoint, and red again on
the stale-HEAD read, before going green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Taskfile.yml

@@ -23,3 +23,8 @@ tasks:
       - golangci-lint run ./...
       - go vet ./...
       - go test ./... -race -count=1
+      # The three commands above cannot see cmd/__PROJECT_NAME__ -- Go's ./...
+      # expansion skips directories whose names start with an underscore, which
+      # is why #1 and #2 shipped green here and red in every generated repo.
+      # This gates the substituted result instead.
+      - bash scripts/tests/generated-child_test.sh