[M0-S9] Write the team workflow: issues, board, branches, stacked PRs #77

Problem

The workflow lived in a deferred plan and a board script nobody had used. The team needs one page that says how an issue becomes a merged PR, the same way as in THD Room Finder and Voyager.

Acceptance criteria

• CONTRIBUTING.md carries the shared Part 1 (plan first, issue types, board, branches, commits, PRs, stacked PRs and the merge order)
• docs/12 explains the jam version: /take, the board views, what each Status means, who reviews
• The deferred automation plan is replaced by what is actually configured; scripts/make_board.py is removed
• The jam plan links its work to the milestone epics

Notes and dependencies

Model: Voyager CONTRIBUTING Part 1, Room Finder docs/contributing/. Stack layer 1 of 3. Suggested owner: Alexander (pipeline, art, scenes, deploy).

Verification

python scripts/check_docs.py passes; a teammate can follow docs/12 from issue to merge without asking.