1.2: Convert all absolute paths to relative

[Acharnite]

Convert absolute paths to relative

The codebase has many hardcoded absolute paths that should use relative paths resolved from the install location (e.g., via \$BROWSER_SKILL_DIR`, `dirname $0`, or gstack-paths`).

Examples found:

1. SKILL.md files — hardcoded ~/.claude/skills/gstack/bin/... paths in bash blocks. These should use \dirname $0`` or an install-dir variable instead.

2. bin/gstack-paths — hardcodes $HOME/.config/opencode/plans and .opencode/plans. The HOME-dependent fallback is fine, but CLI tools should resolve relative to the script location when possible.

3. bin/gstack-gbrain-detect — the TypeScript version uses hardcoded config paths (~/.claude.json, opencode.json, ~/.config/opencode/opencode.jsonc). These should use os.homedir() + path.join or the process working directory.

4. Setup scripts — ./setup references ~/.claude/skills/gstack/ and ~/.config/opencode/skills/gstack/ as install targets. These are install-time decisions, but should accept override via environment variables.

Resolution approach:

• Use path.join(process.cwd(), ...) in TypeScript scripts instead of ~ expansion
• In bash scripts, use SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)" to resolve relative to the script
• Pass install directory through GSTACK_INSTALL_DIR or similar env var
• Avoid bare ~ — use $HOME (which bash expands) instead