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/ci.yaml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [main]
 
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   CARGO_TERM_COLOR: always
   RUST_BACKTRACE: "1"
@@ -28,8 +32,8 @@ jobs:
             ~/.cargo/git
             target
           key: ${{ runner.os }}-cargo-fmt-${{ hashFiles('**/Cargo.lock', '**/Cargo.toml') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-fmt-
+          restore-keys:
+            - ${{ runner.os }}-cargo-fmt-
       - run: cargo fmt --all -- --check
 
   clippy:
@@ -48,8 +52,8 @@ jobs:
             ~/.cargo/git
             target
           key: ${{ runner.os }}-cargo-clippy-${{ hashFiles('**/Cargo.lock', '**/Cargo.toml') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-clippy-
+          restore-keys:
+            - ${{ runner.os }}-cargo-clippy-
       - run: cargo clippy --workspace --all-targets -- -D warnings
 
   test:
@@ -66,8 +70,8 @@ jobs:
             ~/.cargo/git
             target
           key: ${{ runner.os }}-cargo-test-${{ hashFiles('**/Cargo.lock', '**/Cargo.toml') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-test-
+          restore-keys:
+            - ${{ runner.os }}-cargo-test-
       - run: cargo test --workspace --all-targets
 
   keymaps:
@@ -88,8 +92,8 @@ jobs:
             ~/.cargo/git
             target
           key: ${{ runner.os }}-cargo-test-${{ hashFiles('**/Cargo.lock', '**/Cargo.toml') }}
-          restore-keys: |
-            ${{ runner.os }}-cargo-test-
+          restore-keys:
+            - ${{ runner.os }}-cargo-test-
       - name: install Neovim + Vim
         run: |
           set -euo pipefail