fix: resolve all clippy -D warnings across workspace

[bkataru]

Resolves all remaining clippy -D warnings errors across the pawan workspace.

Changes:

• pawan-core/src/agent/mod.rs — Remove unused imports (get_api_key_with_secure_fallback, summarize_args, PawanError), add Value to mod tests import
• pawan-core/src/agent/irc.rs — Restore mut on main where try_receive() is called
• pawan-cli/src/tui/irc_panel.rs — Remove unused mut on worker variables
• pawan-cli/src/tui/model_catalog.rs — Replace sort_by with sort_by_key + Reverse, remove redundant use serde_json;
• pawan-cli/src/tui/queue_panel.rs — Replace .min(3).max(1) with .clamp(1, 3)
• pawan-cli/src/tui/render.rs — Remove needless & in insert_str(&format!(...)), remove redundant use insta;
• pawan-cli/src/tui/slash_handlers.rs — Use .enumerate() on iterator, sort_by_key for case-insensitive sort, checked_div for safe division
• pawan-cli/src/tui/types.rs — Rename ExportFormat::from_str to parse to avoid confusion with std::str::FromStr::from_str
• pawan-cli/tests/tui_types_tests.rs — Fix unused variables in match arms, update from_str → parse references

Verification: All 1643 tests pass, cargo clippy --all-features --all-targets -- -D warnings exits clean.