✨ feat(body): support optional PR template sections

[zrr1999]

Summary

Add requirement-aware PR-body template parsing while preserving the existing required-by-default behavior.

• treat every unmarked H2 in the repository PR template as required;
• allow <!-- pr-body:optional --> before an H2 to make only that section optional;
• accept <!-- pr-body:required --> for explicitness without changing the default;
• require present sections to remain in template order and reject undeclared or duplicate H2 headings;
• fail closed on duplicate template headings, multiple directives for one section, or dangling directives;
• ignore headings and directives inside fenced code blocks;
• keep the existing checklist validation independent of section optionality;
• document the marker contract in the composite action and README.

Existing templates without directives retain their current exact required-heading behavior.

Validation

• extended tests/test_body_cli.py with required-by-default, optional omission/presence, ordering, duplicate/extra heading, fenced directive, invalid-template, and CLI cases;
• repository CI is expected to run formatting, lint/type checks, and the complete pytest suite on this Draft PR.

Notes

This is a backward-compatible validator capability. It does not change zendev's own PR template in this PR; consuming repositories can adopt optional sections independently after this lands.