feat: AI suggestion (#34)

## Feat
* feat(ai): add AI suggestion engine, context gathering, and ghost text
overlay (`v0.3.0`)

## Security & Perf
* fix(ai): restrict background `--help` execution to a hardcoded command
allowlist to prevent RCE
* fix(root): defer context cancellation in goroutine to prevent resource
leaks
* perf(ai): implement LRU eviction and maximum size limit for
`ProviderCache`
* perf(ai): truncate command context output
(`CommandContextProvider.Gather`) to 1000 characters to save tokens

## Bug Fixes
* fix(ai): add mutex synchronization and thread-safe snapshotting to
`AIEngine.RegisterProvider` and `GatherDynamicContext`
* fix(ai): return a copy of fresh `Suggestion` in `AIEngine.Suggest` to
prevent cache mutation
* fix(ai): use length-prefixed encoding in `EnvSnapshot.Hash` to prevent
delimiter collisions
* fix(ai): skip variable assignment lines containing `=` when extracting
Makefile targets
* fix(runner): check `scanner.Err()` and return `nil` on scan errors in
justfile generator
* test(ci): rename `git commit` to `git checkout` in overlay test to
resolve CI `typos` false positive

tests/root/suggestions_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/versenilvis/iris/root"
+	"github.com/versenilvis/iris/spec"
 )
 
 func TestMergeResults(t *testing.T) {
@@ -27,4 +28,23 @@ func TestMergeResults(t *testing.T) {
 			t.Errorf("Expected max 100 suggestions, got %d", len(res))
 		}
 	})
+
+	t.Run("AI Suggestion Promotion", func(t *testing.T) {
+		aiSugg := &spec.Suggestion{
+			Cmd:        "git commit -m \"fix(auth): login bug\"",
+			Desc:       "AI suggestion",
+			Source:     "ai",
+			Confidence: 85,
+		}
+		root.SetCurrentAISuggestion(aiSugg)
+		defer root.SetCurrentAISuggestion(nil)
+
+		res := root.MergeResults("git c", "history")
+		if len(res) == 0 {
+			t.Fatalf("Expected suggestions, got 0")
+		}
+		if res[0].Cmd != aiSugg.Cmd {
+			t.Errorf("Expected AI suggestion at index 0, got %q (confidence %d)", res[0].Cmd, res[0].Confidence)
+		}
+	})
 }