[milten89]

Closes #103

Implements X1 and the CONTRIBUTING items of P4-4:

• CONTRIBUTING.md: a Prerequisites section (.NET 10 SDK or later plus the .NET 8 runtime; Windows with .NET Framework 4.7.2+ for the net472 tests; no targeting pack needed to build), "check the issues and the project, comment before you start", the CLAUDE.md code, testing and performance sections apply to every contributor (AI tools optional), the error-type description fixed to match NotFoundError (sealed, public const string ErrorCode passed to the base constructor, CreateException() override), and PR step 3 with dotnet test … -f net10.0 and xUnit's --filter-class.
• CLAUDE.md: two rules that state existing practice: no parallel API variants; ConfigureAwait(false) in src/ (every one of the 48 awaits there already uses it).
• PR template: dotnet test -f net10.0.
• CODE_OF_CONDUCT.md: the official Contributor Covenant 3.0 text, unchanged except its two notes: reports go to opensource@ochodek.net (created and tested in Create a public contact e-mail alias #101), and the suggested enforcement ladder is kept. Attribution and the CC BY-SA 4.0 license stay.
• .github/ISSUE_TEMPLATE/0-question.yml: a bilingual "Pytanie / Question" form, label question; the 0- prefix puts it first in the chooser (GitHub lists forms alphanumerically).
• SUPPORT.md: where to ask, check NBP's API first for missing data, security through SECURITY.md.
• GITHUB-SETUP: the question label row notes the form applies it.

Checks

• dotnet build (0 warnings), dotnet format --verify-no-changes; the form's YAML parses; links and anchors checked.
• /stop-slop and /miodkuj on the new prose (not on the Covenant text).
• Reviewer: no blocking or should-fix findings; three nits applied.

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