docs: add repository branch governance policy

[josiahking]

Summary

Adds the repository-owned branch-governance policy foundation for EvolvePHP 2 Phase 2.7A.

This PR documents the intended separation between EvolvePHP 2 development on 2.x and preserved EvolvePHP 1 legacy maintenance on master.

The live GitHub default branch remains master. Repository rulesets, branch protection and required status checks are intentionally deferred to the Phase 2.7B external-settings gate.

Changes

• documents 2.x as the designated EvolvePHP 2 development branch
• preserves master as the EvolvePHP 1 legacy line
• prohibits EvolvePHP 2 development from targeting master
• requires explicit approval for EvolvePHP 1 legacy maintenance
• requires agents to verify the requested base branch and exact SHA
• keeps implementation work on task-specific branches
• clarifies support and contribution branch ownership
• adds a dedicated repository-governance documentation-policy test
• records the Phase 2.7A policy foundation in the changelog

Evidence Boundary

This PR does not claim that:

• 2.x is already the GitHub default branch
• repository rulesets are active
• branch protection is active
• status checks are required
• master has been renamed, deleted or replaced
• a main or 1.x branch exists

Those external GitHub changes remain pending Phase 2.7B.

Local Validation

• governance policy test: 6 tests, 45 assertions
• README consistency: 13 tests, 130 assertions
• RFC consistency: 11 tests, 126 assertions
• Documentation suite: 95 tests, 923 assertions
• Architecture suite: 47 tests, 2,869 assertions
• complete root suite: 142 tests, 3,792 assertions
• PHP 8.4 policy execution: 142 tests, 3,792 assertions
• workspace PHPUnit: 6 tests, 18 assertions
• Deptrac: 0 violations and 0 errors
• PHPStan: no errors
• PHP-CS-Fixer: 0 files fixable
• aggregate workspace quality passed
• Composer manifests valid
• diff check clean

Phase 2.7B

After this PR is merged and the 2.x push CI succeeds, Phase 2.7B will separately:

1. configure repository rulesets;
2. verify required status-check identities;
3. change the GitHub default branch from master to 2.x;
4. verify legacy master preservation;
5. update repository documentation only after live settings evidence exists.