Feature: compare_file_contents tool with semantic diffs #1973

Problem

When AI models review diffs, line-based unified diffs can be noisy and token-inefficient. Common scenarios where this hurts:

• JSON/YAML reformatting: A single value change plus auto-formatting creates a huge diff
• Config file updates: Version bumps or reordering keys produce misleading diffs
• CSV/data files: Row shifts make line-based diffs nearly unreadable

Models struggle to identify the actual change amid formatting noise, wasting context tokens and reducing comprehension accuracy.

Proposed Solution

Add a new tool compare_file_contents that:

1. Takes two refs (base and head) plus a file path
2. For supported formats (JSON, YAML, CSV, TOML), produces a semantic diff showing only value changes
3. For unsupported formats, falls back to unified diff
4. Always shows the format used and whether fallback was applied

Example: Semantic vs Line-based

Line-based diff (noisy):

-{"users":[{"id":1,"name":"Alice"},{"id":2,"name":"Bob"}]}
+{
+  "users": [
+    {"id": 1, "name": "Alice"},
+    {"id": 2, "name": "Bobby"}
+  ]
+}

Semantic diff (clear):

users[1].name: "Bob" → "Bobby"

Tool Signature

compare_file_contents(
  owner: string,
  repo: string,
  path: string,
  base: string,    // commit SHA, branch, or tag
  head: string,    // commit SHA, branch, or tag
)

Use Cases

1. Change verification: Model edits a file, uses this tool to confirm only intended changes were made
2. PR review: Quickly understand what actually changed in config/data files
3. Debugging: Compare file across commits without formatting noise

Implementation Notes

• Start behind a feature flag
• Semantic diff enabled by default for supported formats (no opt-out needed initially)
• Pure Go implementation using standard library JSON + yaml.v3
• Supported formats to start: JSON, YAML
• Future: CSV, TOML, other structured formats

Why This Helps Models

• Fewer tokens = more room for reasoning
• Unambiguous output = clearer before/after semantics
• Path notation (e.g., users[1].name) is already familiar to models
• Self-verification = models can check their own edits efficiently