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>

CHANGELOG.md

@@ -10,6 +10,18 @@ changed between versions.
 
 ## [Unreleased]
 
+## [v0.1.2] - 2026-04-29
+
+### Fixed
+
+- `banger update` now writes the freshly-installed binary's commit
+  and built_at fields to `/etc/banger/install.toml`, not the running
+  CLI's. Previously install.toml's `version` was correct after an
+  update but `commit` + `built_at` still pointed at the pre-update
+  binary's identity, which made `banger doctor` raise a false-positive
+  "CLI/install drift" warning on every update. Caught by the v0.1.0
+  → v0.1.1 live update smoke-test.
+
 ## [v0.1.1] - 2026-04-29
 
 ### Added
@@ -125,6 +137,7 @@ root filesystem and network, and exits on demand.
   the swap rather than starting up against an incompatible store.
 - Linux only. amd64 only. KVM required.
 
-[Unreleased]: https://git.thaloco.com/thaloco/banger/compare/v0.1.1...HEAD
+[Unreleased]: https://git.thaloco.com/thaloco/banger/compare/v0.1.2...HEAD
+[v0.1.2]: https://git.thaloco.com/thaloco/banger/releases/tag/v0.1.2
 [v0.1.1]: https://git.thaloco.com/thaloco/banger/releases/tag/v0.1.1
 [v0.1.0]: https://git.thaloco.com/thaloco/banger/releases/tag/v0.1.0