refactor(orchestrator): extract ActivityDriver ABC + driver factory

Stage 4 of the realism migration. Lifts the driver Protocol into a
proper ABC with default plant_file/read_file methods (raise
NotImplementedError), and adds get_driver_for(action) so the
orchestrator worker can dispatch by action shape without isinstance
chains.

SSHDriver now inherits ActivityDriver and implements:
- plant_file: streams base64 via stdin (ARG_MAX-safe, mirrors
  decnet.canary.planter; commit c17b9e0). Honours mtime via touch -d
  so realism-planned files don't all stamp at wall-clock-now.
- read_file: docker exec cat with FileNotFoundError on rc=1, used by
  the upcoming EditAction (stage 3b).

EmailDriver inherits ActivityDriver. Driver alias kept for back-compat
during the migration; removed once realism stages 5-7 land.

decnet/orchestrator/drivers/email.py

@@ -28,7 +28,7 @@ from email.utils import formatdate
 from typing import Any, Optional
 
 from decnet.logging import get_logger
-from decnet.orchestrator.drivers.base import ActivityResult
+from decnet.orchestrator.drivers.base import ActivityDriver, ActivityResult
 from decnet.orchestrator.emailgen.scheduler import EmailAction
 from decnet.orchestrator.emailgen.threads import new_message_id
 from decnet.realism.llm import LLMBackend, LLMTimeout, get_llm
@@ -148,7 +148,7 @@ def _build_eml(
     return msg.as_bytes()
 
 
-class EmailDriver:
+class EmailDriver(ActivityDriver):
     """Concrete driver for :class:`EmailAction`.
 
     Stateless across calls — the LLM backend is constructed once at