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/spawn.rs

@@ -1,4 +1,4 @@
-//! AgentDefinition -- declarative specification for instantiating a
+//! `AgentDefinition` -- declarative specification for instantiating a
 //! subagent from workflow scripts or programmatic calls.
 
 use serde::{Deserialize, Serialize};