community: CODEOWNERS, issue and PR templates, code of conduct, claiming issues; CI on pull requests from forks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

CODE_OF_CONDUCT.md

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+# Code of conduct
+
+FieldCraft is built in the open by people with different backgrounds and levels of experience.
+Everyone taking part — in issues, pull requests, reviews and discussions — is expected to:
+
+- be respectful and assume good intent; criticise work, never people;
+- keep discussion technical, specific and on topic;
+- not harass, insult, demean or discriminate against anyone, publicly or privately;
+- respect the project's decisions on scope, licences and clean-room rules.
+
+The maintainer may edit or remove comments, close threads, and block anyone who does not follow
+this code. Report problems privately to the maintainer through the contact on the GitHub
+profile of [@jyviko](https://github.com/jyviko).
+
+This code is our own, written in the spirit of the
+[Contributor Covenant](https://www.contributor-covenant.org/).