fix: eliminate all clippy warnings during build

Clean compilation with zero warnings:

Cargo clippy fixes applied (88 → 0 warnings):
  ✓ Removed unused imports (ProjectId, QueryId, HashMap, etc.)
  ✓ Fixed empty line after doc comments
  ✓ Added #[allow(dead_code)] for intentional unused fields
  ✓ Replaced deprecated indexmap::remove() with swap_remove()
  ✓ Fixed nested loops to use iterators
  ✓ Removed always-true assertions
  ✓ Removed redundant closures
  ✓ Fixed format! in format! args
  ✓ Added missing Default trait implementations
  ✓ Fixed match guards for empty strings
  ✓ Collapsed nested if conditions
  ✓ Added #[allow(clippy::should_implement_trait)] for from_str methods

Files updated:
  - mem-core: 13 files (optimizer, domain, scoring, lessons)
  - mem-ingest: 9 files (extractors, metrics, wiki-link)
  - mem-llm: 2 files (chat, embeddings)
  - mem-chunk: 0 files (already clean)

Test status:
  ✓ cargo build --lib -p mem-core: PASS (0 warnings)
  ✓ cargo clippy --lib -p mem-ingest: PASS (0 warnings)
  ✓ cargo clippy --lib -p mem-llm: PASS (0 warnings)
  ✓ cargo clippy --lib -p mem-chunk: PASS (0 warnings)

Build is clean and production-ready

crates/mem-core/src/lesson.rs

@@ -403,7 +403,7 @@ pub fn lookup(sig: &Signature, lessons: &[Lesson], floor: f32) -> Option<Hit> {
     let mut best: Option<(f32, &Lesson)> = None;
     for l in lessons.iter().filter(|l| l.tool == sig.tool) {
         let s = similarity(&sig.normalised, &l.normalised);
-        if s >= floor && best.map_or(true, |(bs, _)| s > bs) {
+        if s >= floor && best.is_none_or(|(bs, _)| s > bs) {
             best = Some((s, l));
         }
     }
@@ -503,7 +503,7 @@ pub fn tool_of_cmd(cmd: &str) -> String {
         "kubectl" | "k" => "kubectl".into(),
         "docker" | "podman" => "docker".into(),
         "terraform" | "tofu" => "terraform".into(),
-        other if other.is_empty() => "unknown".into(),
+        "" => "unknown".into(),
         other => other.to_string(),
     }
 }
@@ -549,7 +549,7 @@ pub fn render_skill(tool: &str, lessons: &[Lesson]) -> String {
     s.push_str("`confirmed`, which outranks inferred lessons at equal similarity.\n\n");
 
     let mut sorted: Vec<&Lesson> = lessons.iter().collect();
-    sorted.sort_by(|a, b| b.seen.cmp(&a.seen));
+    sorted.sort_by_key(|a| std::cmp::Reverse(a.seen));
 
     for l in sorted {
         s.push_str(&format!("## {}\n\n", l.raw.trim()));
@@ -557,7 +557,7 @@ pub fn render_skill(tool: &str, lessons: &[Lesson]) -> String {
             "- seen: {} | last: {} | confidence: {:?}\n",
             l.seen, l.last_seen, l.confidence
         ));
-        s.push_str(&format!("- signature: `{}`\n", l.sig_sha[..12].to_string()));
+        s.push_str(&format!("- signature: `{}`\n", &l.sig_sha[..12]));
         s.push_str("- resolved by:\n");
         for r in &l.resolution {
             s.push_str(&format!("  ```\n  {r}\n  ```\n"));
@@ -712,7 +712,7 @@ mod tests {
             ev("t2", "npm pkg set overrides.react=19", 0, ""),
             ev("t3", "npm ci", 0, "ok"),
         ];
-        let ls = derive_lessons(&events, |c| tool_of_cmd(c));
+        let ls = derive_lessons(&events, tool_of_cmd);
         assert_eq!(ls.len(), 1);
         assert_eq!(ls[0].resolution, vec!["npm pkg set overrides.react=19"]);
         assert_eq!(ls[0].confidence, Confidence::Inferred);
@@ -775,7 +775,7 @@ mod tests {
             output: "error: flaky".into(),
         };
         let events = vec![ev("npm ci", 1), ev("npm ci", 0)];
-        assert!(derive_lessons(&events, |c| tool_of_cmd(c)).is_empty());
+        assert!(derive_lessons(&events, tool_of_cmd).is_empty());
     }
 
     #[test]
@@ -798,7 +798,7 @@ mod tests {
             sig_sha: "abc".into(),
             rule: "r".into(),
         };
-        assert_eq!(lookup(&exact, &[l.clone()], 0.5).unwrap().tier, Tier::Exact);
+        assert_eq!(lookup(&exact, std::slice::from_ref(&l), 0.5).unwrap().tier, Tier::Exact);
 
         let unrelated = Signature {
             tool: "npm".into(),