docs: define topic branch lifecycle

[tensornull]

What changed

• Document dedicated short-lived topic branches and keep dev as the clean landing branch.
• Require successful pull-request checks and a squash merge on origin/main before closing a topic lifecycle.
• Define fail-closed --ff-only synchronization of main and dev.

Why

The repository contract did not explicitly define how an agent returns from merged task work to a clean, synchronized dev branch. The new rules make the accepted lifecycle and stop conditions auditable without destructive Git operations.

Validation

• Independent standards review: no findings after revision
• Independent spec review: no findings after revision
• git diff --check
• uv lock --check
• uv run python scripts/check_version.py --require-public-preview-docs
• uv run python scripts/check_secrets.py