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

CONTRIBUTING.md

@@ -3,8 +3,25 @@
 Thanks for helping. Read [AGENTS.md](AGENTS.md) first: it holds the rules every change follows,
 whether a person or an agent writes it.
 
+## Before you start: claim an issue
+
+Every change starts from an issue, and only the maintainer merges.
+
+1. Pick an open issue labelled **help wanted** or **good first issue** that is not labelled
+   **claimed**. If what you want to do has no issue, open one first and wait for a reply.
+2. Comment that you would like to take it. The maintainer assigns it to you and labels it
+   **claimed**; then it is yours. Work on unassigned issues may not be accepted.
+3. Open a draft pull request early, linked with `Closes #N`. An issue with no visible progress
+   for 14 days goes back to the pool.
+
+Issues labelled **in progress** are being worked on by the core team; please don't start on them.
+
+## Making the change
+
 1. Branch from `main`, keep a pull request to one topic.
-2. Run `cargo xtask ci` before pushing; CI runs the same gates.
+2. Before pushing: `cargo fmt --all`, and `cargo clippy -p <crate> --all-targets -- -D warnings`
+   and `cargo test -p <crate>` for the crates you touched. CI runs the full gate on your pull
+   request once the maintainer approves the run.
 3. User-visible behaviour is a command (`crates/engine/src/cmd/`) with tests, including hostile
    parameters.
 4. Meshing and solver algorithms cite the paper they come from in the module docs.
@@ -12,3 +29,7 @@ whether a person or an agent writes it.
    (see [docs/adr/0001-libraries.md](docs/adr/0001-libraries.md)).
 
 By contributing you agree your work is licensed under MIT OR Apache-2.0.
+
+`main` is protected: changes land only through pull requests that the maintainer has reviewed,
+as rebase merges (keep each commit building and passing its tests). Be kind: see the
+[code of conduct](CODE_OF_CONDUCT.md).