docs: require explicit branch discipline

[bankkaroproduct]

What changed

• Requires every project template to declare its GitHub default branch, integration branch, and production deploy trigger.
• Replaces generic main assumptions with project-specific branch-map language across setup, shipping, orchestration, hardening, handoff, review, and checkpoint docs.
• Adds a safe existing-branch cleanup playbook: inventory first, classify keep/merge/park/delete, then delete remote branches only after review.

Why

APMs and agents should not have to guess which branch is the right branch. The framework now makes branch identity a project fact recorded in AGENTS.md, not a default inherited from examples.

Validation

• git diff --cached --check passed.
• Text sweep for stale usually main style wording completed; remaining main mentions are explicit examples or warnings not to assume it.