Add --whitespace-insensitive flag to similarity comparison

[that-github-user]

Problem

Diff similarity in src/scoring/diff-parser.ts uses exact string matching on added lines. Two agents that write semantically identical code but with different indentation (e.g., 2 spaces vs 4 spaces, trailing whitespace) are scored as dissimilar and placed in separate convergence groups. This is especially common when:

• Agents reformat existing code incidentally
• Files use mixed indentation
• Agents add the same logic in different contexts (e.g., inside an if block)

Current behavior

// line comparison is exact
addedLines.add(`${file}:${line}`);

" const x = 1;" and "const x = 1;" are treated as different lines.

Proposed solution

• Add --ignore-whitespace flag to thinktank run (default: false)
• When enabled, normalize lines before adding to the similarity set: trim leading/trailing whitespace and collapse internal runs of whitespace to a single space
• Thread through RunOptions → computePairwiseSimilarity()
• Also pass -w to the underlying git diff call so the raw diffs themselves ignore whitespace (prevents whitespace-only changes from inflating diff sizes)

Acceptance criteria

• --ignore-whitespace normalizes lines in Jaccard similarity computation
• Agents with whitespace-only differences score ≥ 0.95 similarity when flag is set
• Default behavior unchanged (flag is opt-in)
• Unit tests: two diffs differing only in indentation, with and without the flag
• README documents the flag with a note about when to use it (reformatting tasks)

[that-github-user]

Fixed in #121. Added --whitespace-insensitive flag that normalizes whitespace before Jaccard comparison. 4 new tests. Generated by thinktank Opus.