provenance: pass directly through thought::dispatch, remove globals

Provenance now flows as a function parameter through the entire tool
dispatch chain: thought::dispatch → memory::dispatch → store methods.

Removed task_local (TASK_AGENT), thread_local (TASK_PHASE), and env
var (POC_PROVENANCE) from the tool dispatch path. The env var remains
only as a fallback for non-tool paths (CLI commands, digest).

Phase names are passed from knowledge.rs → llm.rs → api.rs, and
api.rs updates the provenance string between steps. No globals needed.

src/hippocampus/store/mod.rs

@@ -37,7 +37,7 @@ pub use parse::{MemoryUnit, parse_units};
 pub use view::{StoreView, AnyView};
 pub use persist::fsck;
 pub use persist::strip_md_keys;
-pub use ops::{TASK_AGENT, set_phase};
+pub use ops::current_provenance;
 
 use crate::graph::{self, Graph};