docs(merge-gate): use jq optional chaining in post-merge snippets (review)

Apply .mergeCommit?.oid and .check_runs[]? so the documented commands don't
abort with a fatal jq error (e.g. 'Cannot iterate over null') when an API
response is missing or unexpected.

Signed-off-by: Sebastian Mendel <info@sebastianmendel.de>

Author: CybotTM

skills/git-workflow/references/merge-gate-watcher.md

@@ -68,11 +68,11 @@ Pitfalls baked in: `grep -c` exits 1 on zero matches (`|| true`); decide hard-fa
 After merge, the base branch (`main`) fires its own runs (CI, release, deploy). To confirm those, query the **commit's** checks keyed on the merge SHA — never filter `gh run list` by `headSha`:
 
 ```bash
-SHA=$(gh pr view $PR --repo $R --json mergeCommit --jq '.mergeCommit.oid')
-gh api repos/$R/commits/$SHA/check-runs --jq '.check_runs[]|{name,status,conclusion}'
+SHA=$(gh pr view $PR --repo $R --json mergeCommit --jq '.mergeCommit?.oid')
+gh api repos/$R/commits/$SHA/check-runs --jq '.check_runs[]?|{name,status,conclusion}'
 gh api repos/$R/commits/$SHA/status      --jq '{state, total:(.statuses|length)}'   # legacy commit statuses (Sonar/codecov)
 ```
 
 `gh run list --json … --jq 'select(.headSha=="'$SHA'")'` is unreliable here: the list window is small and time-ordered, so a still-running `main` job scrolls out behind unrelated activity and the filter returns empty — which then feeds a `gh run view ""` (HTTP 404) and tempts a hand-rolled `sleep`-poll loop that just times out. The check-runs/status API is authoritative and SHA-addressed. For PR-head checks, `gh pr checks $PR --watch` already blocks to completion — prefer it over any custom loop.
 
-**Pre-existing red ≠ your regression.** If a post-merge gate (e.g. SonarCloud "Quality Gate failed" on N Security Hotspots) is red, check the *prior* base commit before owning it: `gh api repos/$R/commits/<prev-sha>/check-runs --jq '.check_runs[]|select(.name=="<gate>")|.conclusion'`. Identical red on the parent + a diff that touched no relevant code = a pre-existing backlog to report, not a regression to fix.
+**Pre-existing red ≠ your regression.** If a post-merge gate (e.g. SonarCloud "Quality Gate failed" on N Security Hotspots) is red, check the *prior* base commit before owning it: `gh api repos/$R/commits/<prev-sha>/check-runs --jq '.check_runs[]?|select(.name=="<gate>")|.conclusion'`. Identical red on the parent + a diff that touched no relevant code = a pre-existing backlog to report, not a regression to fix.