refactor: apply cargo clippy fixes to improve code quality

[whitelonng]

Summary

Applied cargo clippy suggestions to improve code quality and maintainability, reducing warnings from 27 to 2.

Changes

Automatic Fixes (20 suggestions)

• ✅ Redundant return statements removed
• ✅ Collapsible if statements simplified
• ✅ Iterator optimizations (replaced .last() on DoubleEndedIterator)
• ✅ Unnecessary borrows removed
• ✅ Manual implementations of standard methods replaced
• ✅ Other idiomatic Rust patterns applied

Manual Fixes (7 suggestions)

1. Remove unused import: Manager from lib.rs
2. Replace sort_by with sort_by_key for cleaner sorting:

   // Before
   summaries.sort_by(|a, b| b.saved_at.cmp(&a.saved_at));
   
   // After
   summaries.sort_by_key(|s| std::cmp::Reverse(s.saved_at));

3. Replace manual loop counter with enumerate():

   // Before
   let mut kept = 0;
   for index in (0..history.len()).rev() {
       kept += 1;
   }
   
   // After
   for (kept, index) in (0..history.len()).rev().enumerate() {
   }

4. Collapse nested if let into single pattern:

   // Before
   if let Some(event) = parse_event(data) {
       if let Event::Text(delta) = event {
   
   // After
   if let Some(Event::Text(delta)) = parse_event(data) {

Remaining Warnings (2)

Two "too many arguments" warnings remain for functions that would require larger refactoring:

1. call_background_llm (11 parameters) in agent/sessions.rs
2. emit_tool_event (9 parameters) in agent/mod.rs

These functions are candidates for future refactoring using parameter structs, but are deferred to avoid large changes affecting multiple call sites in this PR.

Benefits

• ✅ More idiomatic Rust code
• ✅ Better performance (iterator optimizations)
• ✅ Improved readability and maintainability
• ✅ 92% reduction in clippy warnings (27 → 2)

Test Plan

• ✅ 186 Rust unit tests pass
• ✅ TypeScript type check passes
• All fixes verified to maintain existing functionality