Implement org-balanced voting

Implement "one vote per organisation" for governance decisions, independent of subproject representation.

### Acceptance criteria

- Governance changes: all maintainers across subprojects vote, ⅔ majority, one vote per org
- Steering elections: all org members across subprojects vote, one vote per org
- Maintainer elections: steering committee votes, one vote per org
- Adding/removing subprojects: steering committee, majority, one vote per org
- Subproject technical decisions remain per-person (lazy consensus / simple majority) — explicitly out of scope for org-balancing
- gitvote configuration (or equivalent tooling) updated to support org-weighted rather than per-person tallying, or a documented manual process if tooling can't yet support it
- Rule documented in GOVERNANCE.md with the decision-type table from the restructuring doc

Read GOVERNANCE.md and locate the restructuring document referenced by the acceptance criteria, then inspect the gitvote configuration or equivalent voting process. The work is done when each listed decision type has the specified organisation-level rule, the tooling or manual process is documented, and GOVERNANCE.md contains the decision-type table.

Written by the indexing model from the issue text.