delete dead flat-file journal tool and ephemeral stripping

Journal entries are written to the memory graph via journal_new/
journal_update, not appended to a flat file. Remove thought/journal.rs
(67 lines), strip_ephemeral_tool_calls (55 lines), default_journal_path,
and all wiring. -141 lines.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/mod.rs

@@ -11,7 +11,6 @@ pub mod working_stack;
 // Re-export shared infrastructure from thought
 pub use crate::thought::{ToolOutput, ProcessTracker, truncate_output};
 pub use crate::thought::memory;
-pub use crate::thought::journal;
 
 use crate::agent::types::ToolDef;