feat: clarify and simplify switching between cloud and local

[siillee]

This PR clarifies and simplifies how to switch between cloud and local mode in Claude and Codex integrations.

[github-actions]

A 21-file refactor that somehow shipped without a single bug—clearly the work of someone avoiding real problems. 🎯

🟢 No blocking issues found.

Architecture highlights:

• COGNEE_BACKEND=local correctly scrubs cloud vars from os.environ (not just config), so HTTP hot paths and child processes stay local
• Forced cloud mode is pinned—no silent fallback to local when misconfigured; status line shows ✕ (missing_cognee_base_url) immediately
• Plugin-specific vars (COGNEE_CLAUDE_BACKEND / COGNEE_CODEX_BACKEND) correctly beat the shared COGNEE_BACKEND; cross-plugin vars are properly ignored
• Empty string guards in session-start.py prevent boot attempts when target_url="" (forced cloud, no URL)
• Test coverage is comprehensive: env scrubbing, child process survival, precedence, status line, doctor diagnostics

Code quality:

• Clean separation: _env_file.py scrubs environ, config.py sets _forced_backend flag, status line reads both
• UTF-8 encoding fix for Windows test harness (bonus improvement)
• Documentation is clear and thorough

See inline comments for details.

No fixes needed — ship it.