fix: Make setup.sh work on Linux (#54)

Merging comprehensive setup.sh fixes that resolve critical installation bugs:

**Critical Fixes:**
- ✅ Creates ~/.claude/hooks symlink (hooks were never firing without this!)
- ✅ Creates ~/.claude/skills symlink (skills were never loading!)
- ✅ Creates ~/.claude/scratchpad directory  
- ✅ Fixes PAI_DIR path bug (settings.json symlink pointed to wrong location)
- ✅ Adds Linux support with platform detection (Debian/Ubuntu tested)
- ✅ Fixes prompt display issues for curl piping
- ✅ Git stash/pop for safe re-runs and testing

**Testing:**
- Tested on Debian 13.1 and Ubuntu MATE 24.04
- Comprehensive error handling and backup logic
- Proper user prompts and safety checks

**Credit:**
Huge thanks to @0xsalt for the extensive testing and comprehensive fix!

This PR fixes 6 out of 7 critical bugs (86%) - the remaining context/CLAUDE.md cleanup was added separately.

Related: Addresses issues mentioned in #67 and #62

Author: 0xsalt