fix(ci): version derives from the shipped set; untrack an 18MB binary

Three build-hygiene fixes that turned up while explaining the pathspec.

**version.sh derives from what SHIPPED.** It read bare HEAD, so any commit
moved the version — including one touching only CI or a README. Rules 148
and 149 both specify the pathspec form. Now a denylist, and the direction
is the point: as an allowlist the list must be updated by whoever adds a
directory and nothing fails if they don't, so the failure mode is a changed
artifact keeping its old version silently on a green run. Inverted, new
content counts by default.

android/ is deliberately NOT excluded, and that is the subtle part. This
repo ships TWO artifacts from ONE derivation: android/ is in no server
image, but it is the APK's entire source, and excluding it would stop an
Android-only commit from moving the APK's own version — the silent
downgrade the versioning rework exists to prevent. So the list is the
union: exclude only what ships in neither, and accept that an Android
commit also nudges the server's reported version. Over-inclusion across the
two, which is the harmless direction. roundtable/roundtable-android each
keep tighter lists because they are one-artifact repos; don't copy theirs.

**.dockerignore excluded the wrong CI directory.** It named .forgejo/ and
.github/, neither of which this repo has. Gitea Actions reads .gitea/, so
the one directory that exists was the one not excluded. The "Flutter mobile
client" block had also lost its PATTERN when flutter_client/ was deleted,
leaving a comment describing an exclusion that was not happening — android/
never took its place, so 4.1MB of Gradle project entered the context and
busted the `COPY . .` layer on every Android-only change. bin/ excluded too.

**bin/minstrel was tracked** — an 18MB binary last refreshed by a commit
about web test mocks, and re-dirtied by every `make build` since. Untracked
and ignored; the file stays on disk.

Guards are behavioural rather than textual: they build throwaway repos with
pinned commit timestamps and run version.sh against them, so they break when
the derivation changes rather than when the wording does. Falsified — drop
the .gitea exclusion and the CI-only commit moves the version; add an
android exclusion and an Android commit stops moving it; exclude everything
and a source commit refuses.

One honest note on the refusal test: the script already refused an empty
result via the downstream date check, so the new explicit check improves the
diagnostic ("no commit touches the shipped file set — shallow clone?") and
not the safety. The test pins the property, which is defended in depth.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

.gitignore

@@ -12,6 +12,11 @@
 # Test binary, built with `go test -c`
 *.test
 
+# `make build` output. bin/minstrel was tracked until 2026-09-10 — an 18 MB
+# binary committed by accident, last refreshed by a commit about web test
+# mocks, and re-dirtied by every local build since.
+bin/
+
 # Bundled Android APK + version sidecar (#397). Populated by CI for
 # tag releases; never committed. README in client/ explains the flow.
 client/minstrel.apk