Find every stale worktree, not just the managed ones

- discover worktrees from `git worktree list` on each source checkout, not only
  the worktree root, so hand-made ones, stale registrations and orphaned
  directories are classified too
- normalise each candidate to its main checkout, so a linked worktree in the
  source root cannot offer up the repo's real checkout
- keep locked, mid-rebase and detached-with-unique-commits worktrees, whose
  removal would destroy state nothing else holds
- name the retained branch in every unproven verdict
- add --no-fetch, --json, --include-unmanaged and --include-keep

internal/agent/git_test.go

@@ -42,10 +42,20 @@ branch refs/heads/benvin/foo
 worktree /home/ben/.cache/agentws/repo__detached
 HEAD 3333333333333333333333333333333333333333
 detached
+
+worktree /home/ben/.cache/agentws/repo__gone
+HEAD 4444444444444444444444444444444444444444
+branch refs/heads/benvin/gone
+prunable gitdir file points to non-existent location
+
+worktree /home/ben/.cache/agentws/repo__held
+HEAD 5555555555555555555555555555555555555555
+branch refs/heads/benvin/held
+locked
 `
 	wts := ParseWorktreeList(out)
-	if len(wts) != 3 {
-		t.Fatalf("got %d worktrees, want 3: %+v", len(wts), wts)
+	if len(wts) != 5 {
+		t.Fatalf("got %d worktrees, want 5: %+v", len(wts), wts)
 	}
 	if wts[0].Branch != "main" || wts[0].Path != "/home/ben/src/prodenv/repo" {
 		t.Errorf("wt[0] = %+v", wts[0])
@@ -56,6 +66,104 @@ detached
 	if !wts[2].Detached || wts[2].Branch != "" {
 		t.Errorf("wt[2] = %+v, want detached with empty branch", wts[2])
 	}
+	if wts[3].Prunable != "gitdir file points to non-existent location" {
+		t.Errorf("wt[3].Prunable = %q", wts[3].Prunable)
+	}
+	if !wts[4].Locked || wts[4].Prunable != "" {
+		t.Errorf("wt[4] = %+v, want locked and not prunable", wts[4])
+	}
+}
+
+// A bare "prunable" with no reason still has to read as prunable.
+func TestParseWorktreeListPrunableWithoutReason(t *testing.T) {
+	wts := ParseWorktreeList("worktree /tmp/wt\nHEAD 1111111111111111111111111111111111111111\ndetached\nprunable\n")
+	if len(wts) != 1 || wts[0].Prunable == "" {
+		t.Errorf("ParseWorktreeList = %+v, want one prunable worktree", wts)
+	}
+}
+
+func TestGitInProgressOp(t *testing.T) {
+	srcDir := newTempRepos(t)
+	gitDir, err := GitDir(srcDir)
+	if err != nil {
+		t.Fatalf("GitDir: %v", err)
+	}
+	if op, err := GitInProgressOp(srcDir); err != nil || op != "" {
+		t.Fatalf("clean checkout: op = %q, err = %v", op, err)
+	}
+
+	tests := []struct {
+		marker string
+		dir    bool
+		want   string
+	}{
+		{"MERGE_HEAD", false, "merge"},
+		{"CHERRY_PICK_HEAD", false, "cherry-pick"},
+		{"REVERT_HEAD", false, "revert"},
+		{"BISECT_LOG", false, "bisect"},
+		{"rebase-merge", true, "rebase"},
+		{"rebase-apply", true, "rebase"},
+	}
+	for _, tt := range tests {
+		path := filepath.Join(gitDir, tt.marker)
+		if tt.dir {
+			if err := os.Mkdir(path, 0o755); err != nil {
+				t.Fatal(err)
+			}
+		} else if err := os.WriteFile(path, []byte("x\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+		op, err := GitInProgressOp(srcDir)
+		if err != nil || op != tt.want {
+			t.Errorf("%s: op = %q, err = %v; want %q", tt.marker, op, err, tt.want)
+		}
+		if err := os.RemoveAll(path); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+// Commits that reached a remote-tracking ref are not unique local work; commits
+// made after the push are.
+func TestGitCommitsNotOnRemotes(t *testing.T) {
+	srcDir := newTempRepos(t)
+	if n, err := GitCommitsNotOnRemotes(srcDir); err != nil || n != 0 {
+		t.Fatalf("freshly cloned main: n = %d, err = %v; want 0", n, err)
+	}
+
+	if _, err := runGit(srcDir, "checkout", "-b", "benvin/x"); err != nil {
+		t.Fatal(err)
+	}
+	commitFile(t, srcDir, "a.txt", "a\n", "local a")
+	commitFile(t, srcDir, "b.txt", "b\n", "local b")
+	if n, err := GitCommitsNotOnRemotes(srcDir); err != nil || n != 2 {
+		t.Fatalf("two unpushed commits: n = %d, err = %v; want 2", n, err)
+	}
+
+	if _, err := runGit(srcDir, "push", "origin", "benvin/x"); err != nil {
+		t.Fatal(err)
+	}
+	if n, err := GitCommitsNotOnRemotes(srcDir); err != nil || n != 0 {
+		t.Fatalf("after push: n = %d, err = %v; want 0", n, err)
+	}
+
+	commitFile(t, srcDir, "c.txt", "c\n", "local c")
+	if n, err := GitCommitsNotOnRemotes(srcDir); err != nil || n != 1 {
+		t.Fatalf("one commit past the push: n = %d, err = %v; want 1", n, err)
+	}
+}
+
+func commitFile(t *testing.T, dir, name, content, msg string) {
+	t.Helper()
+	if err := os.WriteFile(filepath.Join(dir, name), []byte(content), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := runGit(dir, "add", "."); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := runGit(dir, "commit", "-m", msg); err != nil {
+		t.Fatal(err)
+	}
 }
 
 // gitSeed sets a repo-local identity so commits work without global config.