Testing infrastructure: AGENTS.md rules + unit test framework + geohash tests (11/11 pass)

- Add AGENTS.md: full project context + mandatory testing rules for AI sessions
- Add tests/unit/ with host-compiled C unit test infrastructure
- Clean stubs approach: ESP-IDF type stubs in tests/unit/stubs/, no source modifications
- Fix geohash.c bit extraction bug (3-byte span) found by unit tests
- test_geohash: 11/11 passing with reference vectors (Munich, NYC, origin, boundaries)

1 files changed, 13 insertions, 0 deletions

diff --git a/tests/unit/stubs/freertos/event_groups.h b/tests/unit/stubs/freertos/event_groups.h
new file mode 100644
index 0000000..28f6403
--- /dev/null
+++ b/tests/unit/stubs/freertos/event_groups.h
@@ -0,0 +1,13 @@
+#ifndef STUBS_FREERTOS_EVENT_GROUPS_H
+#define STUBS_FREERTOS_EVENT_GROUPS_H
+
+#include <stdint.h>
+
+typedef void *EventGroupHandle_t;
+#define BIT0 (1 << 0)
+
+static inline EventGroupHandle_t xEventGroupCreate(void) { return (EventGroupHandle_t)1; }
+static inline uint32_t xEventGroupSetBits(EventGroupHandle_t eg, uint32_t bits) { (void)eg; return bits; }
+static inline uint32_t xEventGroupClearBits(EventGroupHandle_t eg, uint32_t bits) { (void)eg; return bits; }
+
+#endif