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>
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