docs: renumber PRD 0081 -> 0083 across the reconcile chunk

Follows the base PR's renumber (0081 was already taken; main is on 0082). Update
every `PRD 0081` reference in the gateway-attach code, tests, and ADR 0006, plus
ADR 0006's link to the PRD file. No behaviour change.

Refs #516, #519.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_backend_secret_reprovision.py

@@ -162,7 +162,7 @@ class TestFirecrackerReprovision(unittest.TestCase):
 
 
 class TestAttachFlow(unittest.TestCase):
-    """PRD 0081 / #519 review: the base backend owns the reconcile flow and
+    """PRD 0083 / #519 review: the base backend owns the reconcile flow and
     fails hard — gather resources, attempt every running bottle, then raise an
     aggregate if any failed (never silently skip)."""
 
@@ -208,7 +208,7 @@ class TestAttachFlow(unittest.TestCase):
 
 
 class TestFirecrackerAttachPrimitive(unittest.TestCase):
-    """PRD 0081: attach_ca_to_agent_vm pushes the current gateway CA into one
+    """PRD 0083: attach_ca_to_agent_vm pushes the current gateway CA into one
     running agent VM over SSH, failing hard."""
 
     def _run_dir(self, root: Path, ip: str = "10.243.0.3") -> Path: