refactor(util): make render_diff generic (caller supplies side titles); move to root util

Generalize render_diff so it isn't supervise-flavored: the "(current)" /
"(proposed)" side titles are no longer baked in — callers pass `before_title`
and `after_label` (required). Move it from orchestrator/supervisor/util.py to
the base bot_bottle.util alongside sha256_hex, and delete the now-empty
supervisor/util.py.

The supervise callsite (Orchestrator.supervise_respond) assigns
before_title="current", after_label="proposed"; the facade no longer re-exports
render_diff (callers import from bot_bottle.util). Behavior at the supervise
callsite is unchanged.

Full unit suite green (2243).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/util.py

@@ -5,6 +5,7 @@ level deeper, under their backend package."""
 
 from __future__ import annotations
 
+import difflib
 import hashlib
 import ipaddress
 import os
@@ -16,6 +17,31 @@ def sha256_hex(content: str) -> str:
     return hashlib.sha256(content.encode("utf-8")).hexdigest()
 
 
+def render_diff(
+    before: str,
+    after: str,
+    *,
+    label: str = "config",
+    before_title: str,
+    after_label: str,
+) -> str:
+    """Unified diff of `before` vs `after`, with the two sides headed
+    `{label} ({before_title})` and `{label} ({after_label})`. Empty diff (no
+    changes) renders as the empty string. The side titles are the caller's —
+    e.g. "current"/"proposed" for a supervise proposal."""
+    diff = difflib.unified_diff(
+        before.splitlines(keepends=True),
+        after.splitlines(keepends=True),
+        fromfile=f"{label} ({before_title})",
+        tofile=f"{label} ({after_label})",
+        lineterm="",
+    )
+    parts = list(diff)
+    if not parts:
+        return ""
+    return "".join(p if p.endswith("\n") else p + "\n" for p in parts).rstrip("\n")
+
+
 def is_ip_literal(value: str) -> bool:
     try:
         ipaddress.ip_address(value)