refactor(prefs): settings saved as /prefs.json, upstream's ConfigSerializer

NodePrefs is now NodePrefsFields (the fields) plus how they're saved: one
JSON key per field, upstream's keys under "radio", "gps", "repeat", "comp"
and "custom" (so either firmware reads what the other wrote of them),
ours under "solo". Saved atomically (temp file, then renamed), through a
buffer. 2.0-rc's binary /prefs is removed, not converted: 2.0 starts from
the defaults; /new_prefs from before 2.0 is still imported.

From upstream with it: txdelay / direct.txdelay / agc.reset.interval
(prefs and CLI), and the board's own settings through getCustom()
(Board::attachDynamicPrefs) in place of the merge's stopgap adapter.
ConfigSerializer reads values up to 191 characters (was 127): 140-byte
texts, an 80-byte table in hex.

Test: every field round-trips through the JSON byte for byte; the alarm
label tests expect "Once" (the label changed).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/MyMesh.h

@@ -4,7 +4,6 @@
 #include <Mesh.h>
 #include <helpers/ui/DisplayDriver.h>
 #include "Features.h"   // FEAT_RX_POWERSAVE, used by MyMesh.cpp
-#include <helpers/KeyValueStore.h>
 
 // Forward declaration for UITask
 class UITask;
@@ -285,6 +284,9 @@ protected:
   float getAirtimeBudgetFactor() const override;
   int getInterferenceThreshold() const override;
   bool getCADEnabled() const override;
+  int getAGCResetInterval() const override {
+    return ((int)_prefs.agc_reset_interval) * 4000;   // milliseconds
+  }
   int calcRxDelay(float score, uint32_t air_time) const override;
   uint32_t getRetransmitDelay(const mesh::Packet *packet) override;
   uint32_t getDirectRetransmitDelay(const mesh::Packet *packet) override;
@@ -699,15 +701,6 @@ private:
   bool send_unscoped;   // force un-scoped flood (instead of using send_scope)
   char cli_command[80];
   char reply_buf[166];
-  class BoardPrefs : public KeyValueStore {   // MyMesh.cpp
-    MyMesh* _mesh;
-    uint8_t* field(const char* key);
-  public:
-    BoardPrefs(MyMesh* mesh) : _mesh(mesh) { }
-    bool setByKey(const char* key, const char* value) override;
-    bool getByKey(const char* key, char* value, size_t max_len) override;
-  };
-  BoardPrefs _board_prefs;
   uint8_t app_target_ver;
   uint8_t *sign_data;
   uint32_t sign_data_len;