fix(specs): assign spec numbers transactionally

Summary

• Assign spec numbers inside the Specs.create_spec/2 transaction instead of relying on PostgreSQL sequence defaults.
• Lock spec-number allocation and compute the next public number from the current maximum so rolled-back creates do not consume numbers.
• Remove the old specs_number_seq default and add MCP coverage for rollback behavior during spec creation.

Testing

• mix format
• mix test test/hive/mcp/components/tools/create_spec_test.exs test/hive/specs_test.exs
• mix compile --warnings-as-errors
• mix test

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