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>

bot_bottle/orchestrator/service.py

@@ -41,7 +41,6 @@ from .supervisor import (
     TOOL_EGRESS_BLOCK,
     Supervisor,
     render_diff,
-    sha256_hex,
 )
 
 
@@ -216,7 +215,6 @@ class Orchestrator:
             tool=tool,
             proposed_file=proposed_file,
             justification=justification,
-            current_file_hash=sha256_hex(proposed_file),
         )
         self._supervisor.write_proposal(proposal)
         return proposal.id