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/x-media/src/site/pixiv/api.rs

@@ -392,16 +392,31 @@ mod tests {
     use super::*;
     use dotenv::dotenv;
 
+    /// Skips when `PIXIV_REFRESH_TOKEN` is absent or empty (CI without the
+    /// secret must stay green; GitHub Actions exposes an unset secret as an
+    /// empty string, so `is_err()` alone is not enough).
+    fn require_pixiv_token() -> bool {
+        std::env::var("PIXIV_REFRESH_TOKEN")
+            .ok()
+            .filter(|s| !s.is_empty())
+            .is_some()
+    }
+
     #[tokio::test]
     async fn test_fetch() {
         dotenv().ok();
+        if !require_pixiv_token() {
+            eprintln!("skipping: no PIXIV_REFRESH_TOKEN");
+            return;
+        }
         let result = fetch(126839080).await;
         assert!(result.is_ok());
         println!("{:#?}", result);
     }
 
     #[tokio::test]
-    async fn validate_with_bogus_token_fails() {
+    #[ignore = "live network: requires outbound HTTPS to oauth.secure.pixiv.net"]
+    async fn live_validate_with_bogus_token_fails() {
         dotenv().ok();
         // A bogus token must surface as Api error (invalid_grant), not panic.
         let client = PixivAPI::new("bogus_token_for_testing".to_string());