feat: same-Realm peer discovery via UDP and local registry (0.9.10)

Advertise instances with a multicast beacon and a config-dir registry so
same-machine and LAN peers that share a Realm fingerprint can find each
other without TOFU pairing.

Co-authored-by: Cursor <cursoragent@cursor.com>

Makefile

@@ -105,7 +105,7 @@ CORE_SRC = \
 	src/skills.c \
 	src/extensions.c src/plugins.c src/mcp.c src/subagent.c src/attach.c src/clipboard.c \
 	src/pdf_text.c \
-	src/session.c src/runtime.c src/worktree.c src/realm.c \
+	src/session.c src/runtime.c src/worktree.c src/realm.c src/discover.c \
 	src/tools/tools.c \
 	src/tools/patch.c \
 	src/tools/registry.c \
@@ -174,6 +174,7 @@ TESTS = \
 	tests/test_subagent \
 	tests/test_runtime \
 	tests/test_realm \
+	tests/test_discover \
 	tests/test_lowil \
 	tests/test_win_utf8 \
 	tests/test_os_dir \