Add FABLE Pyculator onboarding pilot

[gparadis]

Summary

• Adds a public FABLE Pyculator onboarding guide and links it from the Sphinx docs index.
• Adds structured GitHub issue forms for validation runs, setup problems, and usability observations.
• Adds the Phase 32 onboarding/validation pilot protocol note.
• Adds a scenario/output manifest seed JSON for sanitized FABLE validation evidence.

Phase boundary

This is a mid-phase merge for Phase 32 so the onboarding guide can deploy to public GitHub Pages before P32 closes. P32 remains open after this PR; P32.5 will use first-user feedback to drive focused notebook/docs/interface improvements, and P32.6 will publish modelwright==0.1.0a7 later.

Verification

• .venv/bin/python -m pytest -vv tests/test_examples.py passed with 7 tests.
• .venv/bin/python -m ruff check . passed.
• .venv/bin/python -m pytest passed with 168 passed and 1 skipped benchmark.
• .venv/bin/sphinx-build -b html docs _build/html -W passed and included the onboarding guide.
• Ruby's YAML parser loaded all GitHub issue-template YAML files successfully.
• The manifest seed parsed as valid JSON.
• git diff --check passed.

Closes #192.
Closes #193.
Closes #194.
Closes #195.
Refs #191.
Refs #196.
Refs #197.