feat(extensions): add pipeline extension for end-to-end SDD workflow

[RchrdHndrcks]

Summary

• Adds a pipeline extension under extensions/pipeline/ that orchestrates the full SDD workflow: specify → clarify → plan → tasks → analyze → implement
• Auto-progresses between steps without returning control to the user, pausing only when genuine user input is needed
• Follows the extension format (extension.yml manifest + commands/ directory) as requested in feat(commands): add pipeline command for end-to-end workflow auto-progression #1967 review feedback

Closes #1966
Supersedes #1967

Design Decisions

• Extension, not a core command: Moved from templates/commands/ to extensions/pipeline/ per maintainer feedback
• Declares dependencies: requires.commands lists all 6 core speckit commands it orchestrates
• Single command: speckit.pipeline.run (alias: speckit.pipeline) — keeps the extension minimal and focused

Test plan

• Run /speckit.pipeline.run <feature description> and verify it progresses through all 6 steps automatically
• Verify it pauses during /speckit.clarify for user input and resumes after
• Verify it pauses when /speckit.analyze finds CRITICAL issues
• Verify it stops and reports errors when a step fails
• Test with empty $ARGUMENTS — should ask for feature description

AI Disclosure

This PR was authored with assistance from Claude Code (Claude Opus 4.6). The concept and workflow design are human-directed. The extension was written by Claude and reviewed by the contributor.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new pipeline extension intended to orchestrate the full Spec Kit SDD workflow end-to-end via a single command, aligning with the extension system’s manifest + commands directory structure.

Changes:

• Added extensions/pipeline/extension.yml manifest defining the pipeline extension and its command entrypoint.
• Added extensions/pipeline/commands/pipeline.md command template to auto-run specify → clarify → plan → tasks → analyze → implement with minimal pausing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
extensions/pipeline/extension.yml	Declares the new pipeline extension metadata and the speckit.pipeline.run command (plus alias).
extensions/pipeline/commands/pipeline.md	Command instructions for sequential, mostly hands-free orchestration of the 6-step workflow.

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[Copilot]

As added, this extension is not installable via the default extension catalogs and it’s not bundled into the CLI wheel’s core_pack assets. Unless users manually clone the repo and use specify extension add --dev, they won’t be able to consume it. If the intent is for this to be a usable, shipped extension (and to close #1966), it likely needs a published ZIP artifact (containing extension.yml at the ZIP root) and a catalog entry (core extensions/catalog.json or community catalog.community.json) pointing at that download URL.