Add out-of-scope note: issue trackers must be mainstream

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

.out-of-scope/mainstream-issue-trackers-only.md

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+# Issue tracker integrations are limited to mainstream tools
+
+`setup-matt-pocock-skills` only offers first-class support for **mainstream** issue trackers. Requests to add support for niche, new, or single-vendor experimental trackers are out of scope.
+
+## Why this is out of scope
+
+Every issue-tracker backend hard-codes a CLI shape into the skills (commands, flags, output parsing). Each new backend is permanent maintenance surface — it has to keep working as the tool's CLI evolves, and it has to keep being tested against `/to-prd`, `/to-issues`, `/triage`, and friends. That cost is only worth paying for trackers a meaningful fraction of users actually have.
+
+"Mainstream" is a judgment call, not a numeric bar:
+
+- GitHub, GitLab, and Backlog.md are the kind of tools we'd consider mainstream — broadly known, widely used, well past the experimental phase.
+- A brand-new agent-focused tool with a few hundred GitHub stars is not, no matter how interesting the design.
+
+Stars, age, and download counts are useful signals when making the call but none of them is the rule. The rule is: would a typical engineer recognise this tool and have plausibly chosen it for their team?
+
+The escape hatches for non-mainstream trackers already exist:
+
+- `local markdown` for lightweight in-repo tracking.
+- `other/custom` for users who want to wire something up themselves.
+
+Neither requires the core skills to know about the specific tool.
+
+## Prior requests
+
+- #99 — "Add dex as an issue tracker backend" (dex was ~3 months old and ~300 stars at the time of the request)