feat(dashboard): add draft planning sessions with debounced auto-creation

Introduce a draft lifecycle for planning sessions: typing into the
PlanningModeModal textarea now creates a server-side draft after a
300ms debounce, persisted with status='draft' so the user's in-flight
plan survives modal close/reopen and shows up immediately in the
session list.

- planning.ts: new createDraftSession path; persistSession status
  union widened to include 'draft'; Session gains an explicit title
  field so subsequent updates don't clobber it.
- register-planning-subtask-routes.ts: wires the createPlanningDraft
  POST endpoint that the modal calls on debounce.
- ai-session-store.ts: tracks the draft status across queries so the
  session list and locks behave the same as any active session.
- legacy.ts: client wrapper for createPlanningDraft.
- PlanningModeModal styling, tests, and ModalReentry coverage updated
  for the new flow.
- docs/architecture.md notes the expanded ai_sessions.status lifecycle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture.md

@@ -119,6 +119,7 @@ Concrete references:
   - JSON helpers: `toJson`, `toJsonNullable`, `fromJson`
   - Core schema tables include: `tasks`, `config`, `workflow_steps`, `activityLog`, `archivedTasks`, `automations`, `agents`, `agentHeartbeats`, `task_documents`, `task_document_revisions`, mission hierarchy tables (`missions`, `milestones`, `slices`, `mission_features`, `mission_events`), plugin/routine tables (`plugins`, `routines`), roadmap tables (`roadmaps`, `roadmap_milestones`, `roadmap_features`), insight tables (`project_insights`, `project_insight_runs`), todo tables (`todo_lists`, `todo_items`), `__meta`
   - Migration-created tables include: `ai_sessions`, `messages`, `agentRatings`, `chat_sessions`, `chat_messages`, `runAuditEvents`, `mission_contract_assertions`, `mission_feature_assertions`, `mission_validator_runs`, `mission_validator_failures`, `mission_fix_feature_lineage`
+  - `ai_sessions.status` lifecycle includes `draft` (pre-start planning session), then `generating`, `awaiting_input`, terminal `complete` / `error`
 - **Standalone roadmap model**: `packages/core/src/roadmap-types.ts`, `roadmap-ordering.ts`, `roadmap-store.ts`
   - Roadmap-first entity types (`Roadmap`, `RoadmapMilestone`, `RoadmapFeature`)
   - Pure ordering helpers for contiguous 0-based milestone/feature order and deterministic cross-milestone feature moves