openspec: refresh add-claude-docker-openspec spec/tasks (stale after automate-version-pins)

[dtump]

openspec/changes/add-claude-docker-openspec/ bundles the openspec CLI into the image and is functionally implemented, but its spec/tasks now describe a pinning mechanism that no longer exists:

• tasks.md 1.1/1.3 and specs/openspec-cli/spec.md's "Pin is greppable in Dockerfile" scenario both assume OPENSPEC_VERSION is a Dockerfile ARG. grep '^ARG OPENSPEC_VERSION' Dockerfile returns nothing today.
• The version now lives in pins/openspec.env, generated by update_pins.py — introduced by automate-version-pins (commit ccb5040), which is already implemented on main but not yet archived.
• Artifact files also reference a nonexistent claude-docker/ subdirectory (paths should be repo-root-relative).
• Task 2.x/3.2/3.3 (docker build/run verification) were never actually run against a container in CI or by a human — worth doing once, since nobody has confirmed openspec --version inside a built image.

Suggested fix

Once automate-version-pins is archived, update add-claude-docker-openspec's tasks.md and specs/openspec-cli/spec.md to describe the pins/openspec.env mechanism (or fold the requirement into package-managers, which already documents that openspec installs as part of the same npm install -g --ignore-scripts invocation as claude-code/pnpm — a standalone openspec-cli capability may be partially redundant with that). Then re-verify and archive.

Found while validating open OpenSpec changes for archiving (see #36).