feat(installer): add --uninstall flag to remove GSD files

Adds -u/--uninstall option that cleanly removes all GSD-installed files:
- commands/gsd/ directory (Claude Code) or gsd-*.md files (OpenCode)
- get-shit-done/ directory
- gsd-*.md agent files
- GSD hooks (gsd-statusline.js, gsd-check-update.js/sh)
- GSD entries from settings.json (statusline, hooks)
- GSD permissions from opencode.json (OpenCode only)

Usage: npx get-shit-done-cc --claude --global --uninstall

Closes #236

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -553,6 +553,22 @@ CLAUDE_CONFIG_DIR=/home/youruser/.claude npx get-shit-done-cc --global
 ```
 This ensures absolute paths are used instead of `~` which may not expand correctly in containers.
 
+### Uninstalling
+
+To remove GSD completely:
+
+```bash
+# Global installs
+npx get-shit-done-cc --claude --global --uninstall
+npx get-shit-done-cc --opencode --global --uninstall
+
+# Local installs (current project)
+npx get-shit-done-cc --claude --local --uninstall
+npx get-shit-done-cc --opencode --local --uninstall
+```
+
+This removes all GSD commands, agents, hooks, and settings while preserving your other configurations.
+
 ---
 
 ## Community Ports