Fix semantic diff handling for direct node edits

[orban]

Summary

• add the semantic diff explainer implementation on top of nightshift/bus-factor
• fix direct AGENTS.md / CLAUDE.md changes by treating modified Intent nodes as scoped inputs to themselves
• add a regression test covering CLAUDE.md-only diffs so node-only changes no longer abort under set -u

Assumptions and limitations

• this stays within the narrow first version: standalone CLI script, shell tests, and docs already included in the original feature work
• offline heuristic summaries remain intentionally shallow when ANTHROPIC_API_KEY is unset; this PR only fixes correctness for direct node edits

Validation

• ./tests/test_explain_semantic_diff.sh
• ./tests/test_suggest_updates.sh

Sample output

# Semantic Diff Explainer

Range: main..HEAD
Changed files: 1

## src/AGENTS.md

Summary: The diff changes executable code in this node and deserves semantic review.
Changed files: 1
Confidence: high

Behavioral impact: Behavior likely changed in code paths covered by this node, not just formatting or comments.
Contract impact: Interfaces, invariants, or caller expectations likely changed and should be reviewed against the node guidance.
Internal-only signal: No clear internal-only signal.

Review focus:
- Re-read the node entry point `src/api.ts` because the main workflow changed.
- Inspect `src/api.ts` for user-visible logic changes and edge-case handling.

[orban]

Superseded by #27, which the PR body explicitly acknowledges ('This supersedes the earlier review state on feature/semantic-diff-fix / PR #26'). Closing to consolidate review.