ci: add GitHub Actions workflows with semantic-release auto-versioning

chore: fix all 702 clippy warnings across codebase
- auto-fix 475 via cargo clippy --fix
- fix remaining 227 manually: uninlined_format_args, redundant_closure, match_same_arms,
  underscore_binding, format_push_string, items_after_statements, needless_pass_by_value,
  clone_on_copy, case_sensitive_extension, single_match/let-else, write_with_newline,
  and other clippy lints

src/app/subagent/event.rs

@@ -8,22 +8,27 @@ use serde_json::Value;
 #[derive(Debug, Clone)]
 pub enum SubagentEvent {
     StepCompleted {
-        _step: usize,
-        _output: String,
+        #[allow(dead_code)]
+        step: usize,
+        #[allow(dead_code)]
+        output: String,
     },
     StepFailed {
-        _step: usize,
-        _error: String,
+        step: usize,
+        error: String,
     },
     Completed {
-        _output: String,
+        #[allow(dead_code)]
+        output: String,
     },
     ToolCall {
-        _tool: String,
-        _args: Value,
+        tool: String,
+        #[allow(dead_code)]
+        args: Value,
     },
     ToolResult {
-        _tool: String,
-        _output: String,
+        tool: String,
+        #[allow(dead_code)]
+        output: String,
     },
 }