add pipeline prerequisite validation and guided remediation before running sequencers

[dvschultz]

Summary

Prevent sequencers from failing due to missing prerequisites by adding centralized preflight checks for required artifacts and guided actions in GUI plus structured errors in CLI/MCP.

Source

• File: GMT20260512-000826_Recording.m4a
• Path: /Volumes/Lexar/2026 DADA ZOOM/Week 5/GMT20260512-000826_Recording.m4a
• Transcript time: 01:58:16 - 01:59:50

Evidence

Sequencers work only after cutting, transcribing, and force alignment; needs prerequisite enforcement (GMT20260512-000826_Recording.m4a 7096–7190)

Acceptance Criteria

• Attempting to run a sequencer without required artifacts fails fast with a clear message listing missing prerequisites
• GUI provides one-click actions (or links) to run missing steps
• CLI returns non-zero exit code and prints missing prerequisites in a machine-readable way (e.g., JSON)
• Prerequisite checks are centralized and reused by GUI/CLI/MCP

Implementation Notes

Define artifact contract per stage and shared pipeline-state module; show inline warnings in sequencer panel when unmet.

Extraction confidence: 0.76