setup.sh should set AIPASS_HOME env var and add drone to PATH

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Problem

Drone only works from inside the AIPass directory tree. External projects can't access AIPass services because:

1. No AIPASS_HOME env var — drone's registry finder already supports AIPASS_HOME as a fallback (registry_handler.py line 70-75), but setup.sh never sets it
2. drone not on PATH — On Linux, setup.sh creates a symlink to /usr/local/bin. On Windows, it just prints instructions

Why this matters

The intended architecture: AIPass is installed once, then external projects use drone to access all AIPass services. Without AIPASS_HOME and drone on PATH, this is impossible.

Fix

setup.sh should:

All platforms

• Set AIPASS_HOME env var pointing to the AIPass installation directory

Linux/macOS

• Already creates /usr/local/bin/drone symlink (working)
• Add export AIPASS_HOME=/path/to/AIPass to ~/.bashrc or ~/.profile

Windows

• Add AIPASS_HOME as a user environment variable via setx
• Add .venv/Scripts to user PATH via setx (or create a drone.bat wrapper in a PATH location)

Code already supports it

registry_handler.py already has the AIPASS_HOME fallback in:

• find_registry() — falls back to AIPASS_HOME when CWD walk finds nothing
• get_all_branches() — merges home registry with local project registry
• get_branch_by_name() — checks home registry as fallback

The plumbing is done. Just needs setup.sh to configure it.

Found during Windows 10 setup testing.