feat(skills): add workflow-creator - #105

Adds the workflow-creator prompt-only skill, the remaining scope item of librefang/librefang#6934 (the workflow_create tool itself merged as librefang/librefang#7857).

What it teaches

• When a workflow is the right shape at all, and when a one-off agent_send is better — a workflow that never runs again is a name permanently taken on the daemon.
• The step contract: name / agent / prompt_template required, {{input}} vs output_var vs declared inputs, depends_on switching execution from sequential to topological, and the error_mode / mode vocabularies.
• All four StepAgent shapes — bare string, {"name": …}, {"id": …}, {"type": …} — with guidance on which to reach for. {"type": …} is find-or-spawn, so it is what makes a workflow stand up on an instance where nothing is pre-registered.
• How required_skills behaves: checked after agent resolution and before the prompt is built, resolved against the registry independently of the agent's allowlist, with the three failure classes its error text distinguishes.
• input_schema, including that the type key is param_type rather than type, and what the {{var}}-scanning fallback costs a caller when the schema is omitted.
• Six failure modes with the reason each is rejected, and the durability / no-ownership properties of a registered workflow.

Provenance

Written against the implementation on librefang/librefang@main — WorkflowStep, StepAgent, Workflow::validate, and build_created_workflow — rather than against the issue's sketch of the API, so the ceilings and key spellings are the ones the tool actually enforces.

Verification

The copy pinned in librefang/librefang's registry snapshot is loaded in CI by crates/librefang-runtime/tests/workflow_creator_skill.rs (librefang/librefang#7873): it registers as prompt-only through the same SkillRegistry::load_all path an operator's install takes, clears the load-boundary prompt-injection scan, is order-independent in the rendered skill block, and has its quoted ceilings checked against the live workflow_create schema rather than hardcoded numbers.

SKILL.md only, no skill.toml — matching every other entry here, and deliberate: a skill.toml beside a SKILL.md suppresses the auto-convert, and without a companion prompt_context.md the entire prompt body is dropped with no error.