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/thought/mod.rs

@@ -74,10 +74,11 @@ pub async fn dispatch(
     name: &str,
     args: &serde_json::Value,
     tracker: &ProcessTracker,
+    provenance: Option<&str>,
 ) -> Option<ToolOutput> {
     // Memory and journal tools
     if name.starts_with("memory_") || name.starts_with("journal_") || name == "output" {
-        let result = memory::dispatch(name, args, None);
+        let result = memory::dispatch(name, args, provenance);
         return Some(match result {
             Ok(s) => ToolOutput::text(s),
             Err(e) => ToolOutput::error(e),