Strengthen contradiction check in synthesis workflow

[graspingai-dev]

What changed?

Adds an explicit contradiction pass to the shared synthesis workflow before the dominant story is written.

Why?

A polished synthesis can become overconfident when conflicting evidence is compressed too early. This change makes the review step more explicit without adding a new tool or a large process.

What should reviewers check?

• Does this make the AI output easier to understand and review without making the workflow cumbersome?
• Are contradictions preserved rather than automatically resolved?
• Is the change small enough to become part of the shared default?

Evidence / traceability impact

The change does not alter the source index. It changes how claims in the AI output are checked against conflicting source evidence before they can become reviewed findings.

Supersedes

The current synthesis workflow on main if merged.

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This pull request is intentionally kept as a small, readable example of branch → commit → diff → review → merge for the public starter repository.