thought: create shared cognitive substrate module

New src/thought/ module containing tools and infrastructure shared
between poc-agent and subconscious agents: memory operations, file
tools, bash, context window management.

Currently coexists with agent/tools/ — next step is to wire up both
agent/ and subconscious/ to use thought::dispatch instead of
duplicating the routing logic.

Move dbglog macro to lib.rs so it's available crate-wide regardless
of module compilation order.

src/agent/mod.rs

@@ -1,16 +1,3 @@
-#[macro_export]
-macro_rules! dbglog {
-    ($($arg:tt)*) => {{
-        use std::io::Write;
-        if let Ok(mut f) = std::fs::OpenOptions::new()
-            .create(true).append(true)
-            .open("/tmp/poc-debug.log")
-        {
-            let _ = writeln!(f, $($arg)*);
-        }
-    }};
-}
-
 // agent/ — interactive agent and shared infrastructure
 //
 // Merged from the former poc-agent crate. Contains: