fix: resolve clippy warnings and formatting issues for CI

- Fix doc_markdown warnings in WhatsApp channel
- Fix needless_pass_by_value in cron, health, migration, service modules
- Fix match_same_arms in migration.rs
- Fix too_many_lines in skills/mod.rs
- Fix manual_let_else in tools/file_write.rs
- Apply cargo fmt formatting fixes

All 435 tests pass, clippy clean.

src/channels/mod.rs

@@ -188,7 +188,7 @@ pub fn build_system_prompt(
     }
 }
 
-/// Inject OpenClaw (markdown) identity files into the prompt
+/// Inject `OpenClaw` (markdown) identity files into the prompt
 fn inject_openclaw_identity(prompt: &mut String, workspace_dir: &std::path::Path) {
     #[allow(unused_imports)]
     use std::fmt::Write;