update: refresh install.toml commit + built_at from new binary

After `banger update` swaps binaries, install.toml needs to reflect
the just-installed identity. The previous code passed
buildinfo.Current().{Commit,BuiltAt} into installmeta.UpdateBuildInfo
— but buildinfo.Current() in the running CLI is the OLD pre-swap
binary's identity (we're it), not the staged one. install.toml's
version field got refreshed to target.Version while commit and
built_at stayed pinned at the previous release. `banger doctor`
compares the running CLI's three fields against install.toml's
three fields and so raised a false-positive drift warning on
every update.

Fix: after the swap, exec /usr/local/bin/banger version, parse the
three-line output, and write all three fields to install.toml. If
the exec fails for any reason we fall back to the old behaviour
(version + stale commit/built_at) with a warning, since install.toml
drift is a doctor warning not a broken host — same posture as
before for the failure path.

The parser is split out (parseVersionOutput) and table-tested:
happy path, whitespace-tolerance, missing-field rejection, empty
input rejection, ignoring unrelated lines.

Caught by running v0.1.0 → v0.1.1 live as the first end-to-end
smoke test of the self-update flow, which was the whole point of
that exercise.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/commands_update_test.go

@@ -0,0 +1,79 @@
+package cli
+
+import "testing"
+
+func TestParseVersionOutput(t *testing.T) {
+	cases := []struct {
+		name        string
+		in          string
+		wantVersion string
+		wantCommit  string
+		wantBuilt   string
+		wantErr     bool
+	}{
+		{
+			name: "happy path — three-line shape from banger version",
+			in: `version: v0.1.2
+commit: a0b5c7fa3ca95a37ba99b35280fc75e5647b59e8
+built_at: 2026-04-29T17:34:45Z
+`,
+			wantVersion: "v0.1.2",
+			wantCommit:  "a0b5c7fa3ca95a37ba99b35280fc75e5647b59e8",
+			wantBuilt:   "2026-04-29T17:34:45Z",
+		},
+		{
+			name: "tolerates extra whitespace around the values",
+			in: `  version :   v0.1.2
+  commit :    abc123
+  built_at :  2026-01-01T00:00:00Z`,
+			wantVersion: "v0.1.2",
+			wantCommit:  "abc123",
+			wantBuilt:   "2026-01-01T00:00:00Z",
+		},
+		{
+			name:    "missing commit field is rejected",
+			in:      "version: v0.1.2\nbuilt_at: 2026-01-01T00:00:00Z\n",
+			wantErr: true,
+		},
+		{
+			name:    "empty input is rejected",
+			in:      "",
+			wantErr: true,
+		},
+		{
+			name: "unrelated lines are ignored",
+			in: `banger v0.1.2
+some other diagnostic line: with a colon
+version: v0.1.2
+commit: abc
+built_at: 2026-01-01T00:00:00Z
+`,
+			wantVersion: "v0.1.2",
+			wantCommit:  "abc",
+			wantBuilt:   "2026-01-01T00:00:00Z",
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := parseVersionOutput(tc.in)
+			if tc.wantErr {
+				if err == nil {
+					t.Fatalf("want error, got nil; parsed=%+v", got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got.Version != tc.wantVersion {
+				t.Errorf("Version: got %q, want %q", got.Version, tc.wantVersion)
+			}
+			if got.Commit != tc.wantCommit {
+				t.Errorf("Commit: got %q, want %q", got.Commit, tc.wantCommit)
+			}
+			if got.BuiltAt != tc.wantBuilt {
+				t.Errorf("BuiltAt: got %q, want %q", got.BuiltAt, tc.wantBuilt)
+			}
+		})
+	}
+}