M3.1: action classes and the fast path

Every execution tool call, the driver's and a helper's, gets a verdict
from code before it runs (P3, P4, D6).

- nisse_core.actions, nisse-core's first code: Action and Verdict, and
  reading shell commands statically, segment by segment (cd, wrappers,
  sh -c). What can't be read that way is unknown; a line is as bad as
  its worst command.
- nisse_dev.loop.actions, the rule table: reading, checks and tests are
  harmless; workspace edits, local git and installs on the build VM are
  contained; pushing, other hosts, uploads, publishing, the forge's own
  tools, deleting outside the workspace and wrecking the VM are
  dangerous; changing branches by hand is refused (the harness owns
  branches) with a pointer to the milestone tools.
- A dangerous call isn't run. The item waits on the operator with a
  question rendered from the action, and `allow` lets exactly that
  action run once. A helper can't ask, so it is refused and told to
  hand back. Unknown actions run, logged, until the classifier (M3.2).
- Every verdict is an action_verdict event, which no model sees, and an
  oplog line; `details` shows the counts and every action not simply
  run.
- The property walk gains dangerous_only_when_allowed (checked against
  a mutant that doesn't use the allow up); the slice e2e checks every
  call was judged and none needed an allow.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev/src/nisse_dev/delegation.py

@@ -175,8 +175,11 @@ class Delegation:
         cancel: Cancel | None = None,
         reader: str = "",
         on_turn: Callable[[str], None] | None = None,
+        screen: Callable[[ToolCall], str | None] | None = None,
     ) -> None:
         self._client = client
+        # The fast path (P4): a reason not to run an execution call, or None.
+        self._screen = screen
         self._store = store
         self._workspace = workspace
         # Who acts on the handback, from its capability profile (L9): never
@@ -492,6 +495,16 @@ class Delegation:
                         tokens=spent,
                     )
 
+                refused = self._screen(call) if self._screen else None
+                if refused is not None:
+                    transcript.append(
+                        ToolResult(
+                            call_id=call.call_id,
+                            result=f"not run: {refused}",
+                            success=False,
+                        )
+                    )
+                    continue
                 outcome = self._workspace.dispatch(call.tool, call.arguments)
                 if call.tool in MUTATING_TOOLS and outcome.success:
                     did_work = True