Hoist proc-title prefix to `_def_prefix` const

Make the sub-actor proc-title prefix a single
authoritative constant (`_proctitle._def_prefix`) so
the reap-recognition markers and `xontrib` banner pick
it up automatically — one place to flip the prefix
shape going fwd.

Deats,
- `_proctitle._def_prefix: str = '_subactor'`. New
  module-level const consumed by everything that needs
  to know the prefix.
- `set_actor_proctitle(actor, prefix=_def_prefix)`:
  takes an explicit `prefix` arg (default = the const)
  so callers can override per-spawn if they want.
- Default proc-title format:
  `'tractor[<reprol>]'` → `f'{prefix}[<reprol>]'`
  i.e. `_subactor[<reprol>]` by default.
- `_testing/_reap.py`: cmdline + comm markers source
  the prefix from `_proctitle._def_prefix` instead of
  the hardcoded `'tractor['`. So
  `_is_tractor_subactor()` tracks the const
  automatically.
- `xontrib/tractor_diag.xsh`: `acli.reap` orphan-mode
  banner now interpolates the
  `_TRACTOR_PROC_CMDLINE_MARKERS` tuple directly so
  the human-readable mode line stays in sync if the
  prefix shape changes again.

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tractor/_testing/_reap.py

@@ -90,7 +90,6 @@ keys are caller-defined).
 
 '''
 from __future__ import annotations
-
 import os
 import pathlib
 import re
@@ -99,6 +98,9 @@ import stat
 import sys
 import time
 
+
+from tractor.devx import _proctitle
+
 # `/dev/shm` is the POSIX-shm filesystem on Linux + FreeBSD.
 # macOS uses `shm_open` syscalls without a fs-visible path,
 # so the shm helpers refuse to run there.
@@ -230,9 +232,9 @@ def _read_comm(pid: int) -> str:
 #   while `cmdline` for zombies often reads as empty.
 _TRACTOR_PROC_CMDLINE_MARKERS: tuple[str, ...] = (
     'tractor._child',
-    'tractor[',
+    _proctitle._def_prefix,
 )
-_TRACTOR_PROC_COMM_MARKER: str = 'tractor['
+_TRACTOR_PROC_COMM_MARKER: str = _proctitle._def_prefix
 
 
 def _is_tractor_subactor(pid: int) -> bool:

tractor/devx/_proctitle.py

@@ -52,7 +52,13 @@ except ImportError:
     _stp = None
 
 
-def set_actor_proctitle(actor: 'Actor') -> str | None:
+_def_prefix: str = '_subactor'
+
+
+def set_actor_proctitle(
+    actor: 'Actor',
+    prefix: str = _def_prefix,
+) -> str | None:
     '''
     Set the calling process's proc-title to identify it as a
     tractor sub-actor.
@@ -69,6 +75,6 @@ def set_actor_proctitle(actor: 'Actor') -> str | None:
     if _stp is None:
         return None
 
-    title: str = f'tractor[{actor.aid.reprol()}]'
+    title: str = f'{prefix}[{actor.aid.reprol()}]'
     _stp.setproctitle(title)
     return title

xontrib/tractor_diag.xsh

@@ -488,6 +488,7 @@ def _tractor_reap(args):
         reap,
         reap_shm,
         reap_uds,
+        _TRACTOR_PROC_CMDLINE_MARKERS,
     )
 
     rc: int = 0
@@ -500,9 +501,8 @@ def _tractor_reap(args):
         else:
             pids = find_orphans()
             mode = (
-                'orphans (PPid==1, intrinsic '
-                'cmdline/comm match — `tractor[…]` or '
-                '`tractor._child`)'
+                f'orphans (PPid==1, intrinsic '
+                f'cmdline/comm match — {_TRACTOR_PROC_CMDLINE_MARKERS}'
             )
 
         if not pids: