chore: add upstream remote (input-leap/input-leap) and document sync workflow

[digital-michael]

Goal

Track the upstream Input Leap repo so cherry-picks for Phase 2 are straightforward and we can monitor upstream activity.

Steps

• git remote add upstream git@github.com:input-leap/input-leap.git
• git fetch upstream
• Confirm git log upstream/master --oneline -5 shows current upstream history
• Document the sync command in CONTRIBUTING.md (fetch upstream, do not merge into main)

Acceptance criteria

• upstream remote present in local clone
• Sync workflow documented in CONTRIBUTING.md (docs: add CONTRIBUTING.md — PR workflow, branch conventions, test requirements #9)

Notes

This is a local config step only — no commit needed. The CONTRIBUTING.md entry is the deliverable that lands in the repo.