feat: add find_path and get_file_signatures MCP tools

- find_path: BFS traversal to trace relationship chains between two files (max N hops)
- get_file_signatures: lightweight context mode returning just function/type names
- db.find_path(): bidirectional BFS with shortest-path tracking
- db.get_all_files(): list all files with docs and staleness
- db.get_file_signatures(): return functions list without full doc payload

Inspired by Octocode's GraphRAG path-finding pattern.
Addresses Mike/Dmitry feedback on usable roll-up summaries.

mcp_server.py

@@ -143,6 +143,46 @@ def get_stale_docs() -> str:
     return "\n".join(lines)
 
 
+@mcp.tool()
+def find_path(source: str, target: str, max_depth: int = 4) -> str:
+    """Trace the relationship chain between two files. Shows how file A connects to file B through imports/dependencies. Useful for understanding impact radius and architectural coupling."""
+    db = _get_db()
+    paths = db.find_path(source, target, max_depth)
+    db.close()
+    if not paths:
+        return f"No connection found between {source} and {target} within {max_depth} hops."
+    lines = [f"Connection paths from {source} → {target} ({len(paths)} found):\n"]
+    for i, path in enumerate(paths[:5]):  # Cap at 5 paths
+        chain = " → ".join(path)
+        lines.append(f"  Path {i+1} ({len(path)-1} hops): {chain}")
+    if len(paths) > 5:
+        lines.append(f"  ... and {len(paths) - 5} more paths")
+    return "\n".join(lines)
+
+
+@mcp.tool()
+def get_file_signatures(path: str) -> str:
+    """Get just the function/type signatures for a file — lightweight context without full documentation. Useful when you need a quick map of what a file exports."""
+    db = _get_db()
+    f = db.get_file_signatures(path)
+    db.close()
+    if not f:
+        return f"File not found: {path}"
+    import json
+    try:
+        funcs = json.loads(f["functions"]) if f["functions"] else []
+    except (json.JSONDecodeError, TypeError):
+        funcs = []
+    staleness = " [STALE]" if f["staleness"] == "stale" else ""
+    lines = [f"Signatures for {path}{staleness} ({f['language']}):\n"]
+    if funcs:
+        for fn in funcs:
+            lines.append(f"  • {fn}")
+    else:
+        lines.append("  (no functions extracted)")
+    return "\n".join(lines)
+
+
 @mcp.tool()
 def get_graph_stats() -> str:
     """Get overall knowledge graph statistics — file count, relationship count, staleness."""