feat: embed dsl-reference.md in wfctl binary

The dsl-reference command now falls back to a compile-time embedded copy
of docs/dsl-reference.md when the file isn't found on disk. This means
wfctl dsl-reference works anywhere — CI environments, user machines,
containers — without needing the source repo checked out.

The embedded copy at cmd/wfctl/dsl-reference-embedded.md must be kept
in sync with docs/dsl-reference.md when the reference is updated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: intel352