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/spec/just_test.go

@@ -0,0 +1,38 @@
+package tests
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	_ "github.com/versenilvis/iris/commands/runner"
+	"github.com/versenilvis/iris/spec"
+)
+
+// Verify that the just command generator parses recipes and returns nil on read errors
+func TestJustGenerator(t *testing.T) {
+	tmp := t.TempDir()
+	content := []byte("# build project\nbuild:\n\techo build\n")
+	_ = os.WriteFile(filepath.Join(tmp, "justfile"), content, 0644)
+
+	oldWd, _ := os.Getwd()
+	_ = os.Chdir(tmp)
+	defer func() { _ = os.Chdir(oldWd) }()
+
+	s := spec.Registry["just"]
+	if s == nil || s.Generator == nil {
+		t.Fatalf("expected just spec with generator to be registered in Registry")
+	}
+
+	res := s.Generator([]string{"just", ""}, "just ", "")
+	if len(res) != 1 || res[0].Cmd != "build" || res[0].Desc != "build project" {
+		t.Fatalf("expected recipe build with comment, got %v", res)
+	}
+
+	// Verify missing file returns nil
+	_ = os.Remove(filepath.Join(tmp, "justfile"))
+	resMissing := s.Generator([]string{"just", ""}, "just ", "")
+	if resMissing != nil {
+		t.Fatalf("expected nil when justfile cannot be read, got %v", resMissing)
+	}
+}