ci: auto-derive versionCode from commit count (never reuse a code)

Releases were silently no-op'ing on fdroid: versionCode was a hand-maintained
constant, so a commit that didn't bump it rebuilt the same debug_<vc>.apk, the
publish receiver's anti-downgrade guard refused the duplicate (exit 3 =
"nothing to do"), and the CI went green having shipped nothing. build.yml now
patches ProjectConfig's versionCode from `git rev-list --count HEAD` (minus an
offset keeping it continuous with the manual era, last=91) before assembling,
so every main commit auto-produces a new monotonic code. The ProjectConfig
literal is now just the local-dev default.

buildSrc/src/main/kotlin/ProjectConfig.kt

@@ -359,6 +359,10 @@ const val STRAW_SDK_TARGET = 35
 // vc=19 / 0.1.0-AE — rust pipeline cutover. Extraction via
 // strawcore-core (Sulkta-OSS/strawcore) via the UniFFI wrapper; no
 // NewPipeExtractor in the runtime path.
+// versionCode is AUTO-DERIVED in CI from the git commit count (see
+// .forgejo/workflows/build.yml "Auto versionCode from commit count") so a
+// release can never silently reuse a code. This literal is the local-dev
+// default only; CI overwrites it at build time.
 const val STRAW_VERSION_CODE = 92
 const val STRAW_VERSION_NAME = "0.1.0-CZ"
 const val STRAW_APPLICATION_ID = "com.sulkta.straw"