refactor: merge split instructions back into single file

Copilot CLI only loads the single file at ~/.copilot/copilot-instructions.md
and does not follow relative links to other files. The 5-file split
broke the instructions - only README.md was loaded, missing all the
detailed commands in other files.

This reverts to a single comprehensive file containing all content from:
- README.md (critical rules, navigation)
- quick-reference.md (command cheat sheets)
- git-workflow.md (branching, PRs, deployment)
- code-standards.md (language-specific rules)
- tools.md (scripts, utilities)

All improvements from the refactor are preserved:
- Fixed critical issues (line refs, commands, flags)
- Clearer sections and organization
- Concrete examples
- Better formatting

Symlink updated: ~/.copilot/copilot-instructions.md -> ~/.dotfiles/shell/copilot-instructions.md

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

Author: codepuncher