Windows cross-platform compatibility fixes

[bilersan]

Summary

ctx currently has several assumptions that break on Windows:

1. Path boundary validation — ValidateBoundary uses case-sensitive path comparison, but on Windows, VS Code lowercases drive letters (e.g. c:\) while filepath.EvalSymlinks returns uppercase (C:\). This causes false boundary violations.

2. Project slug generation — ProjectSlug uses OS-native path separators and includes Windows drive prefixes (C:) in slugs, producing platform-dependent identifiers instead of consistent forward-slash paths.

3. Session path parsing — The recall parser uses filepath.Separator and filepath.Join for session paths, but these are always stored with forward slashes regardless of OS.

4. Journal source links — InjectSourceLink and GenerateZensicalToml emit backslash paths on Windows, breaking Markdown links and navigation items.

5. Feed generation — Uses literal \n and .md strings instead of the config.NewlineLF and config.ExtMarkdown constants.

6. Test portability — 50+ tests fail on Windows due to:

   • Missing USERPROFILE env var (os.UserHomeDir reads USERPROFILE on Windows, not HOME)
   • Missing APPDATA env var (XDG fallback reads APPDATA on Windows)
   • t.Setenv("TZ", "UTC") has no effect on Windows (time.Local is not re-read from env)
   • Hardcoded Unix paths in test assertions
   • Unix-specific test patterns: /bin/sh shebang, 0755 permissions, : PATH separator, /tmp/ paths

Proposed Fix

• Case-insensitive path comparison on Windows for boundary validation
• Platform-independent slug generation with filepath.ToSlash and drive prefix stripping
• Forward-slash-only session path operations
• Normalized forward slashes in generated links and nav items
• Use existing platform constants for newlines and extensions
• Cross-platform test helpers: setLocalUTC, USERPROFILE/APPDATA env setup, conditional permission skips, platform-aware PATH separators and binary extensions