ci: adopt gobject-linter as a blocking static-analysis gate

Add gobject-linter (github.com/bilelmoussaoui/gobject-linter) to CI to catch
GObject/GLib correctness and idiom issues.

Config (gobject-linter.toml):
- min_glib_version = 2.80 (libadwaita >= 1.8)
- default_level = error: every enabled rule blocks CI
- type_style ignored: the project deliberately uses plain C scalar types
- untranslated_string ignored: app is not internationalized yet
- use_auto_cleanup downgraded to warn: it mis-fires on floating GVariant
  references (consumed by g_variant_new_tuple/builders) and on objects kept
  alive past the function (the GlobalShortcuts GDBusProxy); converting those to
  g_autoptr would over-unref and crash, so it advises rather than blocks

Workflow (.github/workflows/gobject-linter.yml): runs the official container
image on push/PR, uploads SARIF to code scanning (non-fatal for fork PRs), and
fails the build on any error-level finding.

Code is brought to a green baseline by applying the auto-fixable findings:
- strcmp -> g_strcmp0 (NULL-safe)
- manual g_free/g_strdup -> g_set_str
- manual source removal -> g_clear_handle_id / g_clear_object / g_clear_pointer
- g_string_free(.., FALSE) -> g_string_free_and_steal
- g_idle_add one-shot -> g_idle_add_once (id now stored and cleared in finalize,
  also closing a latent use-after-free if the window is finalized first)
- include ordering and #pragma once

The 37 remaining use_auto_cleanup findings are reported as warnings for
incremental manual review.

1 files changed, 44 insertions, 0 deletions

diff --git a/.github/workflows/gobject-linter.yml b/.github/workflows/gobject-linter.yml
new file mode 100644
index 0000000..403871f
--- /dev/null
+++ b/.github/workflows/gobject-linter.yml
@@ -0,0 +1,44 @@
+name: GObject Lint
+
+# Static analysis of the GObject/GLib C code with gobject-linter.
+# Rules and severities are configured in ./gobject-linter.toml.
+# This is a blocking gate: the "Lint (blocking)" step fails the build on any
+# error-level finding. use_auto_cleanup is configured as a warning and does not
+# block (it mis-fires on floating GVariant references; see gobject-linter.toml).
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  gobject-linter:
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/bilelmoussaoui/gobject-linter:latest
+    permissions:
+      contents: read
+      security-events: write
+    steps:
+      - uses: actions/checkout@v4
+
+      # Produce SARIF for the Security tab and inline PR annotations.
+      # Never fails the job — the blocking gate below decides pass/fail.
+      - name: Analyze (SARIF)
+        run: gobject-linter . --format sarif > gobject-linter.sarif || true
+
+      - name: Upload SARIF
+        uses: github/codeql-action/upload-sarif@v3
+        # Pull requests from forks get a read-only token and cannot upload to
+        # code scanning; don't let that fail the run.
+        continue-on-error: true
+        with:
+          sarif_file: gobject-linter.sarif
+          category: gobject-linter
+
+      # Blocking gate: exits non-zero on any error-level finding.
+      - name: Lint (blocking)
+        run: gobject-linter .