fix: enable mbedTLS ChaCha20-Poly1305 and HKDF for crypto.c

sdkconfig.defaults was gitignored and missing — fresh clones failed to
link due to undefined references to mbedtls_hkdf and mbedtls_chachapoly_*.

- Remove sdkconfig.defaults from .gitignore
- Add sdkconfig.defaults with required mbedTLS options

.gitignore

@@ -2,7 +2,6 @@
 espilon_bot/build/
 espilon_bot/sdkconfig
 espilon_bot/sdkconfig.old
-espilon_bot/sdkconfig.defaults
 espilon_bot/.config
 espilon_bot/.config.old
 

espilon_bot/sdkconfig.defaults

@@ -0,0 +1,5 @@
+# Required by core/crypto.c (ChaCha20-Poly1305 + HKDF)
+CONFIG_MBEDTLS_POLY1305_C=y
+CONFIG_MBEDTLS_CHACHA20_C=y
+CONFIG_MBEDTLS_CHACHAPOLY_C=y
+CONFIG_MBEDTLS_HKDF_C=y