Handle synchronize webhook and add AMENDED bill status

[mfosterw]

Summary

• Adds a new AMENDED bill status for bills invalidated by a PR code update
• Adds PullRequestHandler.synchronize() to handle GitHub's synchronize webhook action (fired when new commits are pushed to an open PR)
• When triggered, the handler updates the stored PR SHA and closes any open bill with AMENDED status; draft bills are not affected
• Modifies Bill.close() to accept an optional status parameter (default CLOSED) enabling reuse for the amended case

Test plan

• TestBill::test_close_amended — bill.close(status=AMENDED) sets status and disables submit task
• TestPullRequestHandler::test_synchronize — open bill is closed with AMENDED status and task disabled
• TestPullRequestHandler::test_synchronize_no_open_bill — gracefully returns (pull_request, None) when no open bill exists
• Full suite: 240/240 tests passing
• All pre-commit hooks passing

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d7cfec9) to head (12e85f7).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #299   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           49        49           
  Lines         2008      2018   +10     
=========================================
+ Hits          2008      2018   +10     

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