ci: add test/clippy workflow and gate live/token tests

The docker workflow only builds/pushes; tests were a local responsibility.
Add .github/workflows/ci.yml with two layers:

- test: cargo fmt --check + cargo clippy --workspace --all-targets -D
  warnings + cargo test --workspace (fully offline, no secrets) on every
  push/PR, including forks.
- live: the #[ignore]d live-network tests plus the pixiv token-gated
  tests, run on schedule / manual dispatch / tag pushes only (fork PRs
  cannot read repository secrets), with PIXIV_REFRESH_TOKEN /
  TWITTER_AUTH_TOKEN injected and continue-on-error for flaky sites.

Test gating (documented in AGENTS.md):
- live-network tests now carry #[ignore = "live network: ..."] (twitter 3,
  bsky 2, pixiv bogus-token 1) and run via -- --ignored live.
- pixiv token tests early-return when PIXIV_REFRESH_TOKEN is absent or
  empty (an unset GitHub secret arrives as ""); test_fetch previously
  failed without a token.

Also fixes the three clippy assertions_on_constants warnings in send.rs
(required for -D warnings).

crates/xmedia-bot/src/send.rs

@@ -155,7 +155,9 @@ impl Task {
             Task::SendMediaSequence { media_batches, .. } => {
                 media_batches.iter().flatten().collect()
             }
-            Task::SendAnimation { animation, .. } => std::slice::from_ref(animation).iter().collect(),
+            Task::SendAnimation { animation, .. } => {
+                std::slice::from_ref(animation).iter().collect()
+            }
             Task::ForwardMessages { .. } => Vec::new(),
         }
     }
@@ -1333,9 +1335,10 @@ mod tests {
     #[test]
     fn oversized_photo_boundary() {
         // The empirical Telegram limit: sum 10000 passes, 10001 fails.
-        assert!(crate::photo::PHOTO_MAX_DIMENSION_SUM == 10000);
-        assert!(6100 + 3900 <= crate::photo::PHOTO_MAX_DIMENSION_SUM);
-        assert!(6300 + 3730 > crate::photo::PHOTO_MAX_DIMENSION_SUM);
+        // Const-block asserts so clippy's assertions_on_constants stays quiet.
+        const { assert!(crate::photo::PHOTO_MAX_DIMENSION_SUM == 10000) };
+        const { assert!(6100 + 3900 <= crate::photo::PHOTO_MAX_DIMENSION_SUM) };
+        const { assert!(6300 + 3730 > crate::photo::PHOTO_MAX_DIMENSION_SUM) };
     }
 
     #[test]