fix(install): auto-install @gsd-build/sdk so gsd-sdk is on PATH (#2385)

Every /gsd-* command shells out to `gsd-sdk query …`, but the SDK was
never installed by bin/install.js — the `--sdk` flag documented in
README was never implemented. Users upgrading to 1.36+ hit
"command not found: gsd-sdk" on every command.

- Implement SDK install in finishInstall's finalize path
- Default on; --no-sdk to skip; --sdk to force when already present
- Idempotent probe via `which gsd-sdk` before reinstalling
- Failures are warnings, not fatal — install hint printed

Closes #2385

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -193,7 +193,7 @@ npx get-shit-done-cc --all --global      # Install to all directories
 
 Use `--global` (`-g`) or `--local` (`-l`) to skip the location prompt.
 Use `--claude`, `--opencode`, `--gemini`, `--kilo`, `--codex`, `--copilot`, `--cursor`, `--windsurf`, `--antigravity`, `--augment`, `--trae`, `--qwen`, `--codebuddy`, `--cline`, or `--all` to skip the runtime prompt.
-Use `--sdk` to also install the GSD SDK CLI (`gsd-sdk`) for headless autonomous execution.
+The GSD SDK CLI (`gsd-sdk`) is installed automatically (required by `/gsd-*` commands). Pass `--no-sdk` to skip the SDK install, or `--sdk` to force a reinstall.
 
 </details>