ci: verify go toolchain in devcontainer runCmd

[devsy-app]

Spec requirement

The devcontainer declares the Go toolchain as its base image
(.devcontainer/devcontainer.json -> "image": "mcr.microsoft.com/devcontainers/go:1.26"),
and the repo is a Go CLI. The devcontainer CI workflow's runCmd verification command is
the spec gate that asserts the declared toolchain is actually present inside the built
container -- but it only checked task prek uv node, omitting go.

Gap found

.github/workflows/devcontainer.yml -- the workflow_dispatch input default and the
DEFAULT_RUN_CMD env both verified the devcontainer-provided tools, but never verified
go itself. A devcontainer build that silently dropped the Go toolchain (e.g. a base-image
regression or a feature override) would pass CI despite breaking the repo's primary build.

Change

Add go to the runCmd verification list in both the workflow_dispatch input default and
the DEFAULT_RUN_CMD env, so the devcontainer CI now asserts the declared Go toolchain is
present:

-default: for c in task prek uv node; do ...
+default: for c in go task prek uv node; do ...

Minimal, one file, two lines.

Verification

• python3 -c "import yaml; yaml.safe_load(open('.github/workflows/devcontainer.yml'))" -> YAML OK
• task cli:format -> clean (no changes)
• task cli:lint:ci -> 0 issues. (0 new issues)
• task cli:test -> only the known pre-existing failures fail
  (pkg/git TestRepoClone*, and e2e TestRunE2ETests which requires external resources);
  both reproduce identically on clean origin/main (verified via git stash baseline) and
  are unrelated to this YAML-only change.
• devcontainer build lock regeneration: not needed -- no features changed, lock file untouched.

This PR was created by an AI agent as part of an automated daily devcontainer spec job.

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[codacy-production]

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