feat: enhance installation system with universal path transformation

Add support for custom global installation paths with automatic context
reference transformation. This enables users to install to any location
while maintaining working context references.

Changes:
- Add universal path transformation for ANY global installation path
  (not just ~/.config/opencode)
- Implement automatic cleanup trap for temp files on exit/crash/interrupt
- Add early validation for --install-dir argument with clear error messages
- Add developer documentation for context reference convention

Technical Details:
- Path transformation detects local vs global by checking if path ends
  in .opencode (local) or not (global)
- Transformation applies to ALL file types during installation
- sed transforms both @ references and shell command paths
- Cleanup trap handles EXIT, INT, and TERM signals

Testing:
- 31 unit and integration tests (100% pass rate)
- All README installation scenarios verified
- Edge cases tested and handled
- Cross-platform compatible (macOS, Linux, Windows Git Bash)

Documentation:
- Added dev/docs/context-reference-convention.md explaining the required
  @.opencode/context/ format and why it's necessary
- Includes examples, validation instructions, and test results

Breaking changes: None
New capabilities: Custom global path support