refactor(core): consume Nordstein.Core as a git submodule

[JabbaKadabra]

Completes the split that #535 set up. Nordstein.Core now lives in its own public repository, NordsteinSoftware/Nordstein.Core, and Proxytrace consumes it as a git submodule at core/. The in-repo staging copy is removed.

The extraction

Run against the merged master with git filter-repo, mapping the pre-move paths (Proxytrace.Common/, Proxytrace.Common.Tests/, Proxytrace.Testing/) onto the current core/ layout — not git subtree split, which follows paths without renames and would have collapsed everything to the single move commit. Result: 28 commits of real history in the new repo, back through the licensing subsystem, the secrets-at-rest retrofit and the original rename to proxytrace, so git blame on Core still points at the change that shaped each line.

Verified standalone in a scratch clone before pushing (no parent directory to inherit from): dotnet build (0/0), dotnet test (198 passed), dotnet pack (4 packages). Two standalone-only fixups landed on top in the Core repo: a .gitignore (Proxytrace's root one didn't come across) and a README/PUBLISHING rewrite for the standalone reality.

Why a submodule (not a sibling checkout or packages)

core/ is a submodule pinned to a specific Core commit. This was chosen deliberately:

• No feed decision required. A sibling ../Core/ checkout can't be seen from a Docker build context, which would have forced release images onto package mode and therefore onto a published feed — dragging the feed/licence/prefix decisions in core/PUBLISHING.md onto the critical path. The submodule keeps core/ inside the build context, so source mode works everywhere, including Docker, with nothing published.
• Proxytrace stays publicly buildable. Nordstein.Core is public (mirrors Proxytrace), so anonymous --recurse-submodules clones and CI's GITHUB_TOKEN both read it — no 401 for outside contributors.
• A Core edit stays a one-build change. Commit in the Nordstein.Core repo, then bump the submodule pointer here.

The reference machinery is unchanged: NordsteinCorePath already defaults to core/ and UseLocalCore flips on when core/Nordstein.Core.sln exists, so source mode, the Dockerfile restore layers and detect-changes see the same core/ paths they always have.

What changed here

Area	Change
.gitmodules + core gitlink	core/ is now a submodule pinned to Nordstein.Core main
CI (ci, e2e, perf, codeql, release)	checkout with submodules: recursive on the 7 jobs that build core/ or the Docker context; the diff-only jobs (changes, secrets, frontend, manual, release meta/release) are left alone
detect-changes	a submodule bump shows in the diff as the bare path core (no trailing slash) plus .gitmodules; both now count as backend changes
Directory.Build.props	dropped the git subtree split / sibling-checkout framing from the comments
docs/code-reuse.md, docs/commands.md	submodule reality + git clone --recurse-submodules

No product code changed.

Verification

• dotnet build core/Nordstein.Core.sln — clean, standalone (through the submodule)
• dotnet build Proxytrace.sln (source mode against the submodule) — 0 warnings, 0 errors
• Core standalone tests: 198 passed (in the extraction check)
• CI's backend (both solutions) and core-package (product against packed .nupkgs) exercise both modes on this PR.

No user-facing change, so no CHANGELOG entry (same as #535).

Follow-ups (not in this PR)

• CI for the Nordstein.Core repo itself — it has no workflows yet; a standalone build/test (and eventually the CI-prerelease pack from PUBLISHING.md) belongs there.
• Package publishing stays deferred: the feed, Core's licence terms, and the Nordstein.* prefix reservation are unchanged open decisions, now genuinely optional since the submodule needs none of them.
• Dependabot for submodules — so Core bumps are raised the way package bumps are.

🤖 Generated with Claude Code

https://claude.ai/code/session_01A8tYyCfGKz2dnvzEX2GTyd