fix(ci): repair sync-amendments workflow and create missing script

[shichengripple001]

Summary

The sync-amendments.yml workflow has been broken since creation:

• YAML syntax error on line 110 — multi-line --body content dropped to zero indentation, breaking the YAML literal block
• Dead CLI commands — node --local --persist --detach no longer exists (now start --detach)
• Missing script — scripts/update-genesis-amendments.ts was never created

This PR fixes all three:

• Fix YAML by using a heredoc (<<'BODY') for the PR body, avoiding shell/YAML escaping issues
• Replace node --local --persist --detach with start --detach
• Add stop cleanup step with if: always() so Docker containers don't leak
• Create scripts/update-genesis-amendments.ts which:
  1. Connects to mainnet (wss://xrplcluster.com) and calls feature RPC for all enabled amendments
  2. Connects to local rippled (ws://localhost:6006) and calls feature RPC for supported amendments
  3. Filters to amendments enabled on mainnet AND supported locally
  4. Appends new entries to the [amendments] block in src/core/compose.ts before # sync:end
  5. Logs skipped amendments (enabled on mainnet but not supported by local binary)

Test plan

• Trigger workflow manually via workflow_dispatch and verify it runs without YAML errors
• Verify the script connects to mainnet and local rippled successfully
• Confirm no new amendments are appended (list should already be current)
• Force-test by temporarily removing an amendment from compose.ts, running the workflow, and verifying it gets re-added

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