Document main branch ruleset evidence

[aisamuraiagent-source]

Summary

Adds public sanitized governance evidence for the manually configured main branch ruleset.

The evidence documents the intended defensive controls around pull-request review, the Static Defensive Validation status check, deletion restriction, linear history, conversation resolution, and deferred deployment or signed-commit requirements.

Scope

• Documentation only.
• Adds docs/governance/main_branch_ruleset_evidence.md.
• Does not modify runtime application code.
• Does not introduce dependencies, backend behavior, scans, secrets, or deployment state.

Validation

• GitHub compare confirms one documentation commit on codex/document-main-ruleset-evidence against main.
• No local runtime validation was run for this PR creation step.