refactor(supervise): hash the proposed file inside Proposal.new; sha256_hex → root util

Move sha256_hex out of orchestrator/supervisor/util.py to the root
bot_bottle.util (it's a generic string hash, not supervise-specific), and have
Proposal.new take the proposed file contents and compute current_file_hash
itself instead of every caller passing sha256_hex(proposed_file).

Every call site set current_file_hash to the hash of the proposed file, and
nothing reads the field except the DB round-trip (schema + _row_to_proposal +
from_dict, all untouched), so folding the hash into the factory is
behavior-preserving and removes the boilerplate. Callers now pass just the
file; the orchestrator no longer imports sha256_hex at all.

Full unit suite green (2243).

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

tests/unit/test_supervise_cli.py

@@ -19,7 +19,6 @@ from bot_bottle.orchestrator.supervisor import (
     TOOL_EGRESS_BLOCK,
     TOOL_GITLEAKS_ALLOW,
     TOOL_EGRESS_TOKEN_ALLOW,
-    sha256_hex,
 )
 
 
@@ -37,7 +36,7 @@ def _proposal(slug: str = "dev", tool: str = TOOL_EGRESS_ALLOW,
     payload = payloads.get(tool, "")
     return Proposal.new(
         bottle_slug=slug, tool=tool, proposed_file=payload,
-        justification=f"needed for {slug}", current_file_hash=sha256_hex(payload),
+        justification=f"needed for {slug}",
         now=now,
     )