Implement org-balanced voting

[gbartolini]

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