DM3.2: Add D9 micro-task contribution guide and PR template

[Copilot]

Contributors had no guidance on how to run D9 numerical scripts reproducibly, format analytic notes, or structure PRs for the DM3 scoreboard workflow.

CONTRIBUTING.md — new D9 section

• Running scripts: install command, full example invocation for collatz_c9_2_sampling.py, output directory setup, seed-locking requirement, connectome sim entry points
• LaTeX / Markdown conventions: inline vs display math syntax, \label/\ref rules, preferred packages, .bib requirement, doc naming scheme (docs/<topic>_<initials>_<YYYY-MM>.md)

.github/PULL_REQUEST_TEMPLATE.md — new file

Pre-populates every PR with grouped checklists:

• Reproducibility — seed recorded, command pasted, outputs in correct folder
• Tests / verification — clean-install smoke test, results match issue, Lean proof sorry-free
• Data — blob size limit, source citations
• Formatting — conventions link, naming convention
• Scoreboard — task-ID prefix in title, scoreboard row linked

# Reproduction command block in every PR:
python scripts/collatz_c9_2_sampling.py \
  --N 1000 --M 50 --window-type dyadic --start 1 --end 1024 \
  --sample-rate 0.1 --seed 42 --output scripts/out/my_run