refactor(pipeline): replace extension with converge-based workflow

Drop the extensions/pipeline Python resolver, command wrappers, and test
suite in favor of a single workflows/pipeline/workflow.yml, per review
feedback that the workflows engine already owns phase orchestration,
ordering, validation, and resume.

Rework the loop around the new speckit.converge command: analyze runs as a
single pre-implement consistency pass (its findings are artifact-level, not
code-level), and a bounded post-implement convergence loop
(implement -> converge, up to 3 cycles) closes any code/spec gaps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: domattioli