feat(planning): start_planning hands back the active plan that already covers the work (#4079)

Step 4 of milestone 415 "An existing plan is found before a new one is made".
A session that could not see an existing plan made a second one beside it.
start_planning and create_milestone now ask first: an ACTIVE milestone in the
project with the same title, or one that reads as the same plan (title, design
and steps against milestone embeddings), is returned with its progress and a
pointer to create_records(milestone_id=...). Nothing is created; force=true
bypasses.

- dedup.find_matching_plan / plan_gate / plan_match_response; access-checked
  before either arm (rule 78), fail-open like the other gates.
- Done milestones never block; the semantic arm needs 200+ chars of candidate.
- kb_plan_match_threshold (default 0.90) is a setting, in the Settings view,
  and pinned against the Python default by test_settings_defaults_agree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_settings_defaults_agree.py

@@ -36,28 +36,32 @@ import re
 import pytest
 
 ROOT = pathlib.Path(__file__).resolve().parents[1]
-_PY = ROOT / "src" / "scribe" / "services" / "plugin_context.py"
+_SERVICES = ROOT / "src" / "scribe" / "services"
 _VUE = ROOT / "frontend" / "src" / "views" / "SettingsView.vue"
 
-# (python constant, vue ref). Hand-written because the pairing is an editorial
-# fact — the names do not share a convention either side could derive — but
-# every entry is asserted to EXIST on both sides, so a rename fails loudly
-# here rather than silently dropping that threshold from the check.
+# (services module, python constant, vue ref). Hand-written because the
+# pairing is an editorial fact — the names do not share a convention either
+# side could derive — but every entry is asserted to EXIST on both sides, so a
+# rename fails loudly here rather than silently dropping that threshold from
+# the check.
 _PAIRS = (
-    ("AUTOINJECT_DEFAULT_THRESHOLD", "kbInjectThreshold"),
-    ("WRITEPATH_DEFAULT_THRESHOLD", "kbWritePathThreshold"),
-    ("RULEHINT_DEFAULT_THRESHOLD", "kbRuleHintThreshold"),
-    ("TOOLRULE_DEFAULT_THRESHOLD", "kbToolRuleThreshold"),
-    ("PROMPTRULE_DEFAULT_THRESHOLD", "kbPromptRuleThreshold"),
+    ("plugin_context.py", "AUTOINJECT_DEFAULT_THRESHOLD", "kbInjectThreshold"),
+    ("plugin_context.py", "WRITEPATH_DEFAULT_THRESHOLD", "kbWritePathThreshold"),
+    ("plugin_context.py", "RULEHINT_DEFAULT_THRESHOLD", "kbRuleHintThreshold"),
+    ("plugin_context.py", "TOOLRULE_DEFAULT_THRESHOLD", "kbToolRuleThreshold"),
+    ("plugin_context.py", "PROMPTRULE_DEFAULT_THRESHOLD", "kbPromptRuleThreshold"),
+    # The plan gate (milestone 415): it blocks a create, so a form showing a
+    # looser bar than the one in force would be the more misleading drift.
+    ("dedup.py", "PLAN_MATCH_DEFAULT_THRESHOLD", "kbPlanMatchThreshold"),
 )
 
 
-def _python_default(name: str) -> float:
+def _python_default(module: str, name: str) -> float:
     m = re.search(rf"^{re.escape(name)}\s*=\s*([0-9.]+)\s*$",
-                  _PY.read_text(), re.M)
+                  (_SERVICES / module).read_text(), re.M)
     assert m, (
         f"{name} is no longer a bare module-level float in "
-        f"services/plugin_context.py. If it moved or was renamed, update "
+        f"services/{module}. If it moved or was renamed, update "
         f"_PAIRS; if it was retired, drop its row — leaving it here checks "
         f"nothing while looking like coverage."
     )
@@ -75,11 +79,11 @@ def _vue_default(ref_name: str) -> float:
     return float(m.group(1))
 
 
-@pytest.mark.parametrize(("constant", "ref_name"), _PAIRS,
-                         ids=[p[0] for p in _PAIRS])
-def test_the_form_shows_the_default_the_server_uses(constant, ref_name):
+@pytest.mark.parametrize(("module", "constant", "ref_name"), _PAIRS,
+                         ids=[p[1] for p in _PAIRS])
+def test_the_form_shows_the_default_the_server_uses(module, constant, ref_name):
     """An untouched control must render the bar actually in force."""
-    server, form = _python_default(constant), _vue_default(ref_name)
+    server, form = _python_default(module, constant), _vue_default(ref_name)
     assert form == server, (
         f"SettingsView shows {form} for {ref_name} while the server defaults "
         f"to {server} ({constant}). An operator who has never set this reads "