refactor: rename firmware from Plus to Solo

- FIRMWARE_VERSION: v1.15-plus.1 → v1.15-solo.1
- FIRMWARE_PLUS_BUILD → FIRMWARE_SOLO_BUILD in all platformio.ini and UITask.cpp
- docs/plus_features/ → docs/solo_features/ (all sub-docs moved)
- README: update all doc links plus new Documentation section
- GitHub Actions: extract GIT_TAG_VERSION in each build job (was missing,
  causing empty FIRMWARE_VERSION in builds); rename output files to
  solo-VERSION-oled.uf2 / solo-VERSION-eink.uf2; release title → "Solo VERSION"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/MyMesh.h

@@ -15,9 +15,9 @@ class UITask;
 #define FIRMWARE_BUILD_DATE "12 May 2026"
 #endif
 
-// Versioning: vX.Y = upstream base, plus.N = fork revision
+// Versioning: vX.Y = upstream base, solo.N = fork revision
 #ifndef FIRMWARE_VERSION
-#define FIRMWARE_VERSION "v1.15-plus.1"
+#define FIRMWARE_VERSION "v1.15-solo.1"
 #endif
 
 #if defined(NRF52_PLATFORM) || defined(STM32_PLATFORM)

examples/companion_radio/ui-new/UITask.cpp

@@ -78,7 +78,7 @@ public:
     display.setTextSize(1);
     display.drawTextCentered(display.width()/2, date_y, FIRMWARE_BUILD_DATE);
 
-#ifdef FIRMWARE_PLUS_BUILD
+#ifdef FIRMWARE_SOLO_BUILD
     int plus_y = date_y + step;
     display.fillRect(0, plus_y - 1, display.width(), lh + 2);
     display.setColor(DisplayDriver::DARK);