Document post-meeting propagation completion gates

[BillWagner]

Summary

Updates the two authoritative post-meeting execution prompts with durable gates learned from the completed C# 9–14 propagation and alpha-numbering audit:

• explicit target-framework, LangVersion N, generated-project, and Roslyn enum proof;
• exact local validation and terminal GitHub check settlement, with only an explicitly permitted ANTLR grammar-validator failure;
• numeric alpha headings, exact non-dry-run renumbering, and second-run idempotence while preserving placeholders on feature PR source trees;
• source-first repair, oldest-to-newest propagation, topology discovery, and no aggregate without a complete authorized feature set;
• guarded rewrites, remote-SHA equality, ledger evidence, and protected checkout/worktree/stash integrity.

These requirements are encoded at the reusable Phase A and Phase B workflow entry points rather than duplicated in normative or conceptual documentation.

Validation

• git diff --check
• prompt front matter, fenced-code balance, and trailing-whitespace checks
• final diff inspection
• confirmed the repository Markdown workflow intentionally excludes .github/ prompt files

No standard text or production workflow files changed.