docs(instructions): mention .gitattributes / forbid dos2unix

[MarkMichaelis]

Background

PR #120 added .gitattributes.template and a first-sync bootstrap so consumer repos get a starter .gitattributes (consumer-owned thereafter). However, .github/copilot-instructions.md says nothing about line endings, so future agents working in a consumer repo may still default to dos2unix or other ad-hoc fixes when a user reports CRLF issues. That is the exact problem PR #120 was meant to head off.

Proposed change

Add a short note to .github/copilot-instructions.md (3-5 lines), either under "Tool Preferences" or as a new "Line Endings" subsection, covering:

• Line endings are governed by .gitattributes.
• Never run dos2unix or other bulk line-ending conversion tools.
• Policy edits go in the consumer's own .gitattributes, which is consumer_owned -- Pull-SDLC.ai.ps1 never overwrites it after first sync.

Acceptance

• New text in .github/copilot-instructions.md covers the three bullets above.
• Validate Instructions workflow stays green.

Out of scope

• Expanding the shipped .gitattributes.template content.
• Generalizing .gitattributes to be union-merged like .gitignore (explicitly rejected during PR feat(sync): upstream-only gitignore markers + .gitattributes.template bootstrap #120 discussion).

Context

Spawned from PR #120 follow-up review.