feat: flexible intake — deferred variables + prepared sessions (#103)

* feat: flexible intake — deferred variables + prepared sessions

Remove blocking intake form modal. Variables are now filled inline during
flow execution or pre-filled via prepared sessions. Adds PATCH /sessions/{id}/variables
endpoint, POST /sessions/prepare for session pre-staging, inline variable prompts
in StepDetail, editable Session Variables panel, and "Prepared for You" dashboard section.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: pass treeData directly to startSession to avoid stale state

setTree(treeData) hasn't committed when startSession runs immediately
after, so tree is still null and getStepsFromTree returns []. This
caused the step detail area to render empty on new session start.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: wire PrepareSessionModal entry point in Flow Library

Add "Prepare session" button (clipboard icon) to grid, list, and table
views for procedural/maintenance flows. Clicking fetches tree intake
fields and account members, then opens PrepareSessionModal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/types/session.ts

@@ -50,7 +50,7 @@ export interface Session {
   path_taken: string[]
   decisions: DecisionRecord[]
   custom_steps: CustomStep[]
-  started_at: string
+  started_at: string | null
   completed_at: string | null
   outcome: SessionOutcome | null
   outcome_notes: string | null
@@ -60,6 +60,8 @@ export interface Session {
   scratchpad: string
   next_steps: string
   session_variables: Record<string, string>
+  prepared_by_id?: string | null
+  assigned_to_id?: string | null
   batch_id?: string
   target_label?: string
 }
@@ -71,6 +73,14 @@ export interface SessionCreate {
   session_variables?: Record<string, string>
 }
 
+export interface PrepareSessionRequest {
+  tree_id: string
+  session_variables?: Record<string, string>
+  assigned_to_id?: string
+  ticket_number?: string
+  client_name?: string
+}
+
 export interface SessionUpdate {
   path_taken?: string[]
   decisions?: DecisionRecord[]