ci: migrate project automation to Just (#442)

* ci: migrate project automation to Just

Centralize build, test, release, dependency, hook, and website commands in Just recipes. Pin workflow tooling, use the repository toolchain, and run the complete validation set in CI.

* ci: codex being codex

* docs: update CLAUDE.md for just task runner migration

Reflect the switch to just recipes for build/test/clippy/fmt, note
dprint replacing raw rustfmt, and add a cross-platform code requirement.

* fix: resolve clippy warnings breaking CI on ubuntu and windows

Use clone() instead of implicit to_string() on already-owned String
values, gate the windows-only unused make_file_at import behind
cfg(posix), and fix unused mut / manual assign-op in the windows-only
localhost test.

* fix: fmt

.github/PULL_REQUEST_TEMPLATE.md

@@ -29,12 +29,12 @@ Please select relevant options.
 - [ ] I formatted the code with `cargo fmt`
 - [ ] I checked my code using `cargo clippy` and reports no warnings
 - [ ] I have added tests that prove my fix is effective or that my feature works
-- [ ] I have introduced no new *C-bindings*
+- [ ] I have introduced no new _C-bindings_
 - [ ] The changes I've made are Windows, MacOS, UNIX, Linux compatible (or I've handled them using `cfg target_os`)
 - [ ] I increased or maintained the code coverage for the project, compared to the previous commit
 
 ## Acceptance tests
 
-wait for a *project maintainer* to fulfill this section...
+wait for a _project maintainer_ to fulfill this section...
 
 - [ ] regression test: ...