chore(template): generic cross-repo example in the PR template

[LukasWodka]

Summary

The pull-request template's cross-repo example named a specific issue in another repository. A placeholder (tracebloc/<repo>#123) says the same thing; the guidance text is unchanged.

Test plan

• Only the three example strings changed (git diff is 2 lines).
• No repository-specific issue reference remains in the file.

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Note

Low Risk
Documentation-only edits to the PR template with no runtime or security impact.

Overview
Updates the GitHub pull request template so cross-repo closing examples use a generic placeholder instead of a real ticket.

In Related, the cross-repo example changes from Closes tracebloc/backend#2364 to Closes tracebloc/<repo>#123. The checklist line for linking tickets keeps the same Closes tracebloc/<repo>#N wording but drops the trailing (backend#2364) hint. No change to the actual linking rules or check behavior—only example text.

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[saadqbal]

Good fix — backend#2364 was a real, if closed, internal ticket sitting in a template that gets copied org-wide. Genericizing the Related-section example to tracebloc/<repo>#123 is clean and matches the placeholder style README already uses.

One nit, non-blocking: the checklist line's trailing (backend#2364) wasn't really an "example" — it's the actual ticket that mandated the closing-ref rule, cited the same way in Makefile:648 and closing-ref-gate.py's docstring. It's still alive in both of those, so nothing traceability-wise is actually lost, but dropping it from the template means a reader of this file no longer sees why the checklist item exists. Worth a second's thought on whether that was intentional, otherwise fine as-is.

[LukasWodka]

Intentional, yes. This repo is public and the template is copied into every repo in the org, public ones included, so a real internal ticket number in the checklist line surfaces the same way the Related-section example did — it was the same class of leak, just one line down. The provenance you point at (the Makefile line and the gate's docstring) lives in the private tooling repo, which is where a reader who needs the "why" would look anyway; the public template only needs to state the rule.

Happy to add a neutral pointer like "see the closing-ref gate" if you'd rather the checklist item still hint at its origin — say the word and I'll push it. Otherwise this is ready for your approval as-is.