Implement the backend: store/bus/runmgr core

Details (original message):
Implement the backend: store/bus/runmgr core, mock and LLM runners (agent-native client), HITL three-branch routing, subagent runs, builtin and declarative HTTP tools, ingest parsers (md/txt/docx/pdf-text), knowledge fulltext, layered settings, profiles, plugin host, REST+SSE API, single-binary entrypoint; e2e and unit tests.

go.mod

@@ -0,0 +1,26 @@
+module agent-system-backend
+
+go 1.27
+
+replace agent-system-core => ./deps/agent-system-core
+
+replace agent-native => ./deps/agent-native
+
+require (
+	agent-native v0.0.0-00010101000000-000000000000
+	agent-system-core v0.0.0-00010101000000-000000000000
+	modernc.org/sqlite v1.60.1
+)
+
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/ledongthuc/pdf v0.0.0-20260907135840-6c8c28e0e8a0 // indirect
+	github.com/mattn/go-isatty v0.0.24 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	golang.org/x/sys v0.48.0 // indirect
+	modernc.org/libc v1.77.1 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.12.1 // indirect
+)