Add PR checks pipeline (fmt, clippy, test) and split CI config

- Split .woodpecker.yml into .woodpecker/check.yml (PR checks) and .woodpecker/deploy.yml (deploy on push to main)
- Add basic handler test to src/main.rs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.woodpecker/check.yml

@@ -0,0 +1,20 @@
+when:
+  - event: pull_request
+
+steps:
+  - name: fmt
+    image: rust:1.84
+    commands:
+      - rustup component add rustfmt
+      - cargo fmt -- --check
+
+  - name: clippy
+    image: rust:1.84
+    commands:
+      - rustup component add clippy
+      - cargo clippy -- -D warnings
+
+  - name: test
+    image: rust:1.84
+    commands:
+      - cargo test