docs: merge GUIDELINES.md and ARCHITECTURE.md into unified GUIDELINES.md

Combine the two monolithic guideline documents into a single file organized
by topic (Design Principles, Command Conventions, Status Model, Mutation Flow,
Output and Rendering, Error Handling, Git Internals, Commit Conventions).

Key improvements over the originals:
- Bold imperative leads on every subsection for scannability
- Code examples for the two main footguns (error() before throw, gitLocal() exit code)
- Classification check order code skeleton
- Critical rules inlined in CLAUDE.md (always in context without a file read)
- error() before throw added to critical rules (has actual violations)

CLAUDE.md updated with critical rules section and references to GUIDELINES.md.
ARCHITECTURE.md removed (content absorbed into topical sections).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: henrikje