ci: improve workflows with stricter checks, caching, and idempotency

CI improvements:
- Add concurrency group to cancel redundant runs on same branch
- Add cargo cache to fmt job (was missing entirely)
- Add timeout-minutes: 5 to fast jobs (fmt, clippy, test) for fail-fast
- Change RUST_BACKTRACE from 'short' to '1' for better CI debuggability
- Pin push trigger to branches: [main] to avoid double-firing on PR merges
- Test minimum supported Neovim (0.11.0) instead of latest patch (0.11.3)

Release improvements:
- Enforce RUSTFLAGS: -D warnings at env level and in matrix rustflags
- Remove redundant checkout in release job (only needs env vars)
- Add release notes generation from git log since previous tag
- Make release creation idempotent (check if release exists before creating)
- Fix release_id not being set when release already exists

.gitea/workflows/release.yaml

@@ -57,8 +57,8 @@ jobs:
             ~/.cargo/git
             target
           key: ${{ runner.os }}-cargo-release-${{ matrix.target }}-${{ hashFiles('**/Cargo.lock') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-release-${{ matrix.target }}-
+          restore-keys:
+            - ${{ runner.os }}-cargo-release-${{ matrix.target }}-
 
       - name: Build nuwiki-ls
         env: