docs: add non-interactive install flags for Docker/CI

Documents --global/-g and --local/-l flags for use in Dockerfiles,
CI pipelines, and scripts where interactive prompts aren't possible.

Closes #9

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -22,6 +22,15 @@ npx get-shit-done-cc
 
 That's it. Works on Mac, Windows, and Linux.
 
+### Non-interactive Install (Docker, CI, Scripts)
+
+```bash
+npx get-shit-done-cc --global   # Install to ~/.claude/
+npx get-shit-done-cc --local    # Install to ./.claude/
+```
+
+Use `--global` (`-g`) or `--local` (`-l`) to skip the interactive prompt.
+
 Verify: `/gsd:help`
 
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