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

docs/features.md

@@ -45,6 +45,10 @@ Tasks carry status (`todo` → `in_progress` → `done`/`cancelled`), priority
 - **Milestones** — Ordered stages within a project. A milestone is also the home of a
   **plan** — its body holds the design (Goal/Approach/Verification) and its child
   tasks are the steps. Completion percentage is shown on the project page.
+  Milestones are searchable by meaning (`search(content_type="milestone")`), and an
+  agent starting a plan (`start_planning`, `create_milestone`) is handed the active
+  milestone that already has its title or reads as the same plan, so steps are added
+  there rather than to a parallel plan. The match threshold is in Settings.
 - **Kanban view** — `/projects/:id` groups tasks by milestone in a column layout with
   status-advance buttons on the cards.