[feat] Add pipeline extension

[domattioli]

What

Adds a pipeline extension that chains the Spec Kit phases
(specify -> clarify -> plan -> tasks -> analyze -> implement) into a single
guided, tailorable invocation with one interactive clarify gate.

Why

Taking a feature from description to implemented change currently means issuing
each /speckit.* command by hand and hand-resolving analyze findings in between.
This extension collapses that to one command with a single human checkpoint, while
still driving the stock /speckit.* commands (it orchestrates them, it does not
reimplement them).

What is included

• speckit.pipeline.run -- the orchestrator (clarify gate + analyze/fix loop, up to 3 cycles).
• speckit.pipeline.preview -- dry-run printer for the resolved phase plan.
• A pure-Python deterministic phase resolver with strict validation (exit codes 10-14),
  bash + PowerShell wrappers, and a stdlib unittest suite.
• --skip / --add flags; insertable constitution and checklist phases.
• Agent-neutral -- no dependency on any specific AI agent, model, or vendor tooling.

Note for maintainers

Opened as a proposal to include pipeline as a first-party extension.
If you would prefer it live as a community extension instead, happy to move
it to a standalone repo and submit via the Extension Submission issue
template -- just say the word.

Catalog files are intentionally left untouched per the publishing guide.