Add encoding discipline rule to operational-constraints (#256)

* Add encoding discipline rule to operational-constraints

Templates that post comment / reply / description bodies via gh or az rest are vulnerable to silent character corruption (mojibake) on Windows PowerShell 5.x. Adds Rule 10 with the temp-file + UTF-8-without-BOM pattern, the WriteAllText recipe for PowerShell 5.x, an explicit warning against round-tripping through gh pr view --jq | Out-File, and a verify-after-posting step.

Fixes #255

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR #256 review: correct PS5 cmdlet defaults and fix heredoc example

Out-File defaults to UTF-16LE in PowerShell 5.1, not Windows-1252 (Set-Content is the one that defaults to ANSI). Reworded to give per-cmdlet defaults so the rule is authoritative.

Replaced the collapsed-onto-one-line heredoc pseudo-example with a properly-terminated multi-line block, switched the filename to body.md (since gh pr edit --body-file consumes Markdown), and added a note that body.json is appropriate only for JSON-consuming APIs like az rest.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Quote @body.json in az rest example for PowerShell compatibility

Bare @body.json risks being parsed as a splat token in PowerShell.

Quoting it is harmless in bash and required in PowerShell.

Addresses PR #256 review feedback.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>