feat(workflows): expose workflow source directory to steps via context.workflow_dir and SPECKIT_WORKFLOW_DIR

[rhuss]

Summary

Expose the resolved source directory of a workflow definition to steps, enabling workflows to reference sibling files (extension manifests, adapter scripts, etc.) without requiring the user to pass the path redundantly.

Motivation

As discussed in #3445, workflows loaded from a file path (specify workflow run ~/dev/my-project/setup.yml) have no way to discover where their own YAML file lives. This forces users to pass the source directory twice:

SPEX_SOURCE=~/dev/my-project specify workflow run ~/dev/my-project/setup.yml

WorkflowDefinition.source_path is already captured when loading from a file, but it is never propagated to steps.

Proposed solution

1. Template expression: {{ context.workflow_dir }} - the parent directory of the workflow definition file, available in all expression contexts (step configs, conditions, etc.)

2. Environment variable: SPECKIT_WORKFLOW_DIR - set automatically for shell steps, giving shell commands direct access without template interpolation.

3. Value semantics:

   • For file-loaded workflows: parent directory of the source YAML file
   • For installed-by-ID workflows: the installation directory (.specify/workflows/<id>/)
   • For string-loaded workflows: None/empty (no source path available)
   • On resume: the original source directory is preserved (persisted alongside the workflow copy)

Edge cases

• Resume: The definition is reloaded from .specify/workflows/runs/<run_id>/workflow.yml, so the persisted copy's path differs from the original. The original workflow_dir must be persisted in state so resume restores the correct value.
• String-loaded workflows: source_path is None; context.workflow_dir should be empty string or None, and SPECKIT_WORKFLOW_DIR should not be set.

References

• How to reference sibling files from a workflow's source directory? #3445 (original discussion)
• How to reference sibling files from a workflow's source directory? #3445 (comment) by @mnriem suggesting this as a feature request