docs: add gururmm one-time setup step to ONBOARDING

Documents bash scripts/install-hooks.sh requirement after cloning gururmm.
Explains the sqlx migration checksum / CRLF root cause so the step makes
sense and doesn't get skipped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/ONBOARDING.md

@@ -27,6 +27,21 @@ When you open Claude Code for the first time on a new machine, Claude will ask w
 
 After that, every session log and git commit is attributed to you.
 
+### GuruRMM repo — one-time setup per machine
+
+The GuruRMM repo (`projects/msp-tools/guru-rmm/`) requires one extra step after cloning or first use. Run this from the repo root:
+
+```bash
+bash scripts/install-hooks.sh
+```
+
+This does three things permanently:
+- Points git at `scripts/hooks/` so pre-commit checks run automatically (and stay current as hooks evolve — no re-install after updates)
+- Sets `core.autocrlf=false` and `core.eol=lf` for this repo (prevents sqlx migration checksum drift from Windows CRLF line endings)
+- Sets `core.autocrlf=false` globally on this machine
+
+**Why this matters:** sqlx verifies migration files by sha384 hash. A file committed with CRLF line endings hashes differently than the same file with LF — the server sees the mismatch and refuses to start. The `.gitattributes` file handles new commits automatically; this command configures the git client for existing checkouts.
+
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 ## The slash commands (most important daily tools)