ci: pin CI jobs to the arm64 runners, not the shared ubuntu-latest alias

ci.yml's four jobs (lint, tests, audit, gui check) still used
runs-on: ubuntu-latest. Since the Gitea runners were relabeled (see
the previous ci(release) commit), that alias now resolves only to
hostinger-runner — the single native amd64 machine release.yml routes
the x86_64 build legs to. Every CI push was piling onto that one
runner alongside the release jobs, and it could not keep up: run 73
(triggered by the previous commit) shows 'Set up job' taking 15m22s
and then every following step failing in 0s, including deterministic
ones like cargo fmt --check that have nothing to do with runner
architecture.

Pins all four ci.yml jobs to linux-arm64 (the two oracle-runner
replicas), matching the explicit-label convention release.yml already
uses, and freeing hostinger-runner to do only what it was provisioned
for.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VAyr8d7HuAdUGB6aA5xmDD

.gitea/workflows/ci.yml

@@ -26,7 +26,7 @@ env:
 jobs:
   lint:
     name: Format and clippy
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     env:
       CARGO_TARGET_DIR: /opt/hostedtoolcache/cargo-target/guara-lint
     steps:
@@ -45,7 +45,7 @@ jobs:
   # Linux-only runner (ARM64).
   test:
     name: Tests (Linux)
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     env:
       CARGO_TARGET_DIR: /opt/hostedtoolcache/cargo-target/guara-test
     steps:
@@ -78,7 +78,7 @@ jobs:
 
   audit:
     name: Dependency audit
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     steps:
       - uses: actions/checkout@v7
       - uses: dtolnay/rust-toolchain@stable
@@ -92,7 +92,7 @@ jobs:
 
   gui:
     name: Check desktop app (Linux)
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     env:
       CARGO_TARGET_DIR: /opt/hostedtoolcache/cargo-target/guara-gui
     steps: