fix(types): sweep invalid-parameter-default across core modules

Add `| None` to ~250 params across 30 files that were annotated as bare
`str`, `list`, `dict`, `int`, `Callable`, etc. but defaulted to None.

This is a big typechecking fix. each one cascades, making ty stop
narrowing those vars to None and clearing downstream
unresolved-attribute / not-subscriptable / invalid-argument-type errors.

Two type bugs surfaced and fixed during the sweep:

1. Lowercase `callable` (builtin function) used as type annotation in
   28 callback params in agent_init.py + run_agent.py. `callable | None`
   isn't valid. Fixed to `Callable` (typing).

2. String forward-ref with `| None` (`"IterationBudget" | None`) is
    wrong because `|` can't OR a str with NoneType. Fixed to
   `Optional["IterationBudget"]`.

Also:
- Configure ty to exclude tests/ via [tool.ty.src] in pyproject.toml
  (tests are ~57% of diagnostics, lowest-value typing target)

ty diagnostics: 13,290 -> 4,648 (core only, tests excluded)
Tests: 496 passed, 0 failed

tools/memory_tool.py

@@ -957,10 +957,10 @@ def _missing_old_text_error(store: "MemoryStore", target: str, action: str) -> s
 
 
 def memory_tool(
-    action: str = None,
+    action: str | None = None,
     target: str = "memory",
-    content: str = None,
-    old_text: str = None,
+    content: str | None = None,
+    old_text: str | None = None,
     operations: Optional[List[Dict[str, Any]]] = None,
     store: Optional[MemoryStore] = None,
 ) -> str: