Add E2E integration tests with testdata fixtures

[Ilyes512]

Parent: #42 (architectural review)

Problem

Test coverage is broad in unit-test breadth (~37 *_test.go files) but thin on end-to-end flows. The existing tests in pkg/cmd/*_test.go exercise individual commands, but:

• There is no integration test that drives specs use github:foo/bar ./out against a fixture template through the actual cobra command tree.
• There is no canonical testdata/ of sample templates shared across packages. Each test rolls its own ad-hoc template via t.TempDir() and os.WriteFile.
• pkg/util/git/ integration tests are gated with //go:build integration and require network access — appropriate, but the rest of the surface has no equivalent harness.
• Build tag conventions are fine; the gap is in fixture sharing and CLI-level coverage.

Proposed direction

1. Create testdata/ at the repo root (or internal/testdata/) with at least these canonical fixtures:

   • minimal/ — one file, two variables, no conditionals.
   • conditional/ — variables that gate other variables, plus directory-level conditionals via empty path segments.
   • computed/ — at least three computed values with an interdependency graph that exercises the topo sort.
   • hooks/ — pre-use and post-use hooks, both inline (project.yaml) and file-based (hooks/*.sh).
   • verbatim/ — .specsverbatim file plus a binary asset and a file with literal {{ that must not be parsed.
   • delimiters/ — uses __delimiters: { left: "[[", right: "]]" } to verify custom delimiters end-to-end.

2. Add pkg/cmd/integration_test.go (build tag //go:build integration or just standard tests if they don't need network) that drives the cobra command tree:

   • Execute against an in-process fixture (using the local-path source) with stdin/stdout/stderr pipes.
   • Assert on the produced filesystem tree (via fs.WalkDir + golden files or cmp.Diff).
   • Cover: template download (against a local path), template use, specs use (one-shot), template list, template upgrade, template validate.

3. Refactor existing unit tests to consume the shared fixtures where it reduces churn. Don't force a single style — unit tests that exercise one focused branch should stay narrow.

4. CI: keep the existing go test -race ./... lane. Add an integration lane (no network — uses local-path fixtures only) so CI catches end-to-end breakage.

Acceptance criteria

• testdata/ exists with the six fixtures listed above (or a documented subset, justified).
• At least one E2E test per command in pkg/cmd that drives the cobra tree end-to-end against a local-path fixture.
• Integration tests run without network access (no real GitHub clones).
• CI runs both unit and integration lanes on every PR.
• Existing tests still pass; intermittent or filesystem-leaky tests are flagged or fixed.

Out of scope

• Replacing the unit test suite.
• Network-dependent E2E tests against real GitHub (that lane already exists for pkg/util/git).

References

• pkg/cmd/testhelpers_test.go — current shared helpers (small)
• pkg/util/git/ — example of build-tag-gated integration tests
• Architecture docs sample patterns: docs/content/docs/architecture/template-engine.md, computed-values.md