ci(link): gate pull requests into main on fmt, clippy, and test

This repo had no pull_request workflow at all — release.yml runs only
after merge, so its `cargo fmt --check` gate was the first thing to see
new code, and an unformatted commit killed the release job before it
could build, tag, or publish (run for 2301c57).

Add pr-checks.yml, mirroring release.yml's gates in the same order so a
green PR implies the release clears its own gates:

  cargo fmt --check
  cargo clippy --locked --all-targets -- -D warnings
  cargo test --locked

One job, not three: bootstrapping the toolchain costs more than the
checks, so parallel jobs would pay it three times for no wall-clock win.
Cargo registry and target/ are cached on Cargo.lock. The crate is
already clean at `-D warnings`, so clippy starts green.

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -25,6 +25,7 @@ network-facing component, which is what keeps the game unreachable from the inte
 | Path | What |
 |------|------|
 | `sidecar/` | The Rust sidecar crate — terminates the loopback link to the shard, exposes WS + REST to the website. See [`sidecar/README.md`](sidecar/README.md). |
+| `.gitea/workflows/pr-checks.yml` | Gates every PR into `main` on `cargo fmt --check`, `cargo clippy -D warnings`, and `cargo test`. |
 | `.gitea/workflows/release.yml` | Builds + releases the sidecar binary (Linux + Windows) on every merge to `main`. |
 
 ## Build & run
@@ -42,6 +43,17 @@ cargo run --release
 every merge to `main` (conventional-commit versioning). See [`sidecar/README.md`](sidecar/README.md)
 for configuration and the wire protocol.
 
+Before that, `.gitea/workflows/pr-checks.yml` runs the same gates on every pull request into `main` —
+`cargo fmt --check`, `cargo clippy --all-targets -- -D warnings`, then `cargo test --locked`. Run them
+locally before pushing and the PR will be green:
+
+```bash
+cd sidecar
+cargo fmt                                        # or --check to just report
+cargo clippy --locked --all-targets -- -D warnings
+cargo test --locked
+```
+
 ## Deployment & compatibility
 
 The plugin ([RunicGateway/servuo-plugins](https://gitea.whitlocktech.com/RunicGateway/servuo-plugins))