docs(contexts): prevent hard-wrapped GitHub prose

[keli-wen]

Summary

Add a canonical source-format contract for GitHub Issue, Pull Request, Discussion, and comment bodies so future contributors and agents do not hard-wrap remote prose at 80 columns.

Route the contract through AGENTS.md, CLAUDE.md, both quantmind-dev skill copies, the pull-request workflow, the development-context index, label guidance, and the Issue/PR templates. Add regression coverage for every route, template marker, and mirrored skill reference.

The root cause was generation-side prose wrapping, not Ruff or another repository lint rule. This keeps Python's 80-column configuration and repository Markdown formatting unchanged while applying no-wrap normalization only to GitHub bodies.

Related Issue

No issue applies; this is a focused contributor-governance fix requested during Issue maintenance.

Verification

• python -m pytest --no-cov -q tests/test_contexts.py — 6 passed, 50 subtests passed.
• bash scripts/verify.sh — Ruff, BasedPyright, 5 import contracts, and 292 tests passed; total coverage 88.55%.
• Commit and pre-push hooks passed without bypassing verification.

Checklist

• The title uses English Conventional Commit format: type(scope): summary.
• The related issue or design discussion is linked when applicable.
• bash scripts/verify.sh passes.
• Every applicable live-network component smoke test passes, or this PR states why none applies.
• Public behavior has focused tests, an example, and documentation where applicable.
• The PR is complete, small, and contains no unrelated changes.

No live-network component or runtime public behavior changes; live smoke tests and a usage example are not applicable.