docs: document rebase-before-push as soft recommendation

[smartwatermelon]

Summary

• Documents rebasing onto the latest base branch before pushing/updating a PR as a soft, non-blocking process habit — addresses the recurring "This branch is out-of-date with the base branch" GitHub banner.
• Adds the guidance under the pre-push section of docs/WORKFLOW-DEEP-DIVE.md (since the pre-push hook doesn't currently check this) and a corresponding row in the Known Gaps & Limitations table.
• Explicitly scoped as a habit, not a gate: no blocking pre-push check added, and the doc calls out when to skip it (branch already current, shared/collaborative branches, branches under active review).
• Also reformats the file's Markdown tables with proper GFM alignment padding — the existing tables were failing markdownlint's MD060 rule even on main before this change touched them; fixing this was required to get a clean local pre-commit run.

Test plan

• pre-commit (markdownlint) passes locally
• Local pre-push review (adversarial-reviewer + codebase reviewer) both PASS
• Manually verified the #known-gaps--limitations anchor link resolves correctly

Closes #48

https://claude.ai/code/session_019yrvtEbtQxBxDmZ4Fu9GrU