feat: per-user auto-memory with ResponseContext

Three memory channels: hidden tool (sol.memory.set/get in scripts),
pre-response injection (relevant memories loaded into system prompt),
and post-response extraction (ministral-3b extracts facts after each
response). User isolation enforced at Rust level — user_id derived
from Matrix sender, never from script arguments.

New modules: context (ResponseContext), memory (schema, store, extractor).
ResponseContext threaded through responder → tools → script runtime.
OpenSearch index sol_user_memory created on startup alongside archive.

Cargo.toml

@@ -25,4 +25,11 @@ regex = "1"
 anyhow = "1"
 chrono = { version = "0.4", features = ["serde"] }
 url = "2"
-ruma = { version = "0.12", features = ["events", "client"] }
+ruma = { version = "0.12", features = ["events", "client", "markdown"] }
+libsqlite3-sys = { version = "0.30", features = ["bundled"] }
+deno_core = "0.393"
+deno_ast = { version = "0.53", features = ["transpiling"] }
+deno_error = "0.7"
+tempfile = "3"
+reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"] }
+uuid = { version = "1", features = ["v4"] }