fix(plugins): complete steward rename across bundled plugins; clear lint debt

The fabledscryer->steward rename had only ever reached host_agent. The other
five bundled plugins (http, snmp, traefik, unifi, docker) still imported
`from fabledscryer.*` (package no longer exists) and read FABLEDSCRYER_* env
vars — so every one of them was broken at import since the original rebrand.
CI stayed green only because none are enabled by default and migrations don't
import plugin modules. Now that they version in-tree, complete the rename:
- fabledscryer.* -> steward.* imports across all five plugins
- FABLEDSCRYER_* -> STEWARD_* in plugin migration env.py files
- author/repository/homepage + user-facing 'Fabled Scryer' strings -> Steward
- snmp/scheduler.py: also drop dead `now`/datetime; record_metric from steward

Adds tests/test_no_legacy_names.py — fails if 'scryer'/'roundtable' ever
reappear in shipped code (the drift bit twice; this stops a third time).

Also clears pre-existing ruff lint debt (unused imports, semicolon statements,
mid-file import) surfaced by the new lint lane.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/plugins/host_agent/test_agent_build_payload.py

@@ -1,3 +1,4 @@
+import urllib.error
 from unittest.mock import patch
 from plugins.host_agent import agent as a
 
@@ -55,10 +56,6 @@ def test_build_payload_wraps_samples():
     assert payload["samples"][0]["cpu_pct"] == 5.0
 
 
-import urllib.error
-from unittest.mock import MagicMock, patch
-
-
 def test_post_payload_success():
     from plugins.host_agent import agent as a
     captured = {}

tests/plugins/host_agent/test_agent_ring_buffer.py

@@ -3,7 +3,9 @@ from plugins.host_agent.agent import RingBuffer
 
 def test_ring_buffer_preserves_order():
     rb = RingBuffer(maxlen=3)
-    rb.push(1); rb.push(2); rb.push(3)
+    rb.push(1)
+    rb.push(2)
+    rb.push(3)
     assert list(rb.drain()) == [1, 2, 3]
     assert len(rb) == 0
 
@@ -17,7 +19,8 @@ def test_ring_buffer_drops_oldest_when_full():
 
 def test_ring_buffer_drain_clears_and_is_atomic():
     rb = RingBuffer(maxlen=5)
-    rb.push("a"); rb.push("b")
+    rb.push("a")
+    rb.push("b")
     out = list(rb.drain())
     rb.push("c")
     assert out == ["a", "b"]

tests/plugins/host_agent/test_install_template.py

@@ -2,7 +2,6 @@
 import subprocess
 from pathlib import Path
 
-import pytest
 from jinja2 import Environment, FileSystemLoader
 
 from plugins.host_agent.routes import _agent_version, AGENT_SOURCE_PATH