feat: three-state sync planner with warning semantics

- Plan is a pure function (inventory, manifest, disk facts, injected path
  mapping) emitting exactly six actions: deploy, update, skip+warn,
  leave+warn, remove, keep — deterministic order, no filesystem access
- full 3x3 matrix (disk unmodified/user-modified/never-managed x upstream
  added/changed/removed) table-tested plus missing-disk restore cases,
  a mixed fixture and a purity/determinism guarantee test
- executor applies plans item by item (atomic deploy/remove per item),
  rewrites the manifest once, appends timestamped SKIP/LEAVE lines with
  item name and reason to the sync log
- sync now plans before mutating: unmanaged occupants block new deploys,
  upstream deletions remove only unmodified copies, local modifications
  always win and are warned about

internal/target/opencode.go

@@ -6,6 +6,7 @@ package target
 import (
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/m3tam3re/agent-lib/internal/lockfile"
 )
@@ -39,3 +40,32 @@ func (o *OpenCode) DeployPath(typ, name string) (path string, ok bool) {
 		return "", false
 	}
 }
+
+// ScanTargetDirs lists every item currently present in the target dirs,
+// keyed "<type>/<name>" — the raw material for detecting unmanaged occupants.
+func (o *OpenCode) ScanTargetDirs() map[string]bool {
+	out := map[string]bool{}
+	skillsDir := filepath.Join(o.Home, ".agents", "skills")
+	if entries, err := os.ReadDir(skillsDir); err == nil {
+		for _, e := range entries {
+			if e.IsDir() {
+				out[lockfile.TypeSkill+"/"+e.Name()] = true
+			}
+		}
+	}
+	for typ, dir := range map[string]string{
+		lockfile.TypeCommand: filepath.Join(o.Home, ".config", "opencode", "commands"),
+		lockfile.TypeAgent:   filepath.Join(o.Home, ".config", "opencode", "agents"),
+	} {
+		entries, err := os.ReadDir(dir)
+		if err != nil {
+			continue
+		}
+		for _, e := range entries {
+			if !e.IsDir() && strings.HasSuffix(e.Name(), ".md") {
+				out[typ+"/"+strings.TrimSuffix(e.Name(), ".md")] = true
+			}
+		}
+	}
+	return out
+}