L2: permission mode propagation across forward/calendar/adopt + PUT endpoint + tests

P1 - forward inherits mode:
- doForward: when creating a new target session, use InheritedMode from
  the source session (plan parent → plan child, cannot escalate)
- enforcement snapshot set from receiving agent

P1 - calendar_events gets permission_mode column:
- Added to 3 migration sites (sqlite init, pg init, incremental alter)
- CalendarEvent model gains PermissionMode field
- CreateCalendarEvent / UpdateCalendarEvent normalize + persist the field
- resolveCalendarSession: on new session → write event's mode;
  on reuse → ModeAtMost(cur, eventMode), prevents escalation
  (plan Agent's reminder fires into a workspace session = bypass)
- SendCalendarMail now takes permMode and threads it through
- fireEvent passes event.PermissionMode to all delivery paths

P1 - AdoptPlatformSession explicitly writes default mode:
- Writes DefaultPermissionMode + enforcement on adopt, instead of
  relying on DB column default (avoids silent drift on schema changes)

P2 - PUT /sessions/{id}/permission endpoint:
- New handler UpdateSessionPermission (auth required, access check)
- Registers PUT route alongside existing budget/alias endpoints
- Broadcasts session_update on change
- Does NOT refresh enforcement (design: snapshot at creation)

P3 - Tests (24 new cases):
- permission_mode_test.go: InheritedMode (6 cases), SetSessionPermissionMode
  roundtrip, dirty value fail-closed, NormalizePermissionMode, calendar event
  roundtrip/dirty/update, adopt writes default mode, plan escalation guard,
  3-level inheritance chain
- defaultsession_test.go: budget regression, permission mode regression
  (pins created=false → no reset on reuse)

Deploys with: bash deploy/redeploy-gateway.sh --skip-tests
Schema migration: auto via addMissingColumns (new column default 'workspace')

gateway/internal/db/migrations/init_sqlite.sql

@@ -433,6 +433,10 @@ CREATE TABLE IF NOT EXISTS calendar_events (
     -- 按 occurrence 比相等则精确：AdvanceRecurrence 改了 event_time 就再触发，
     -- 没改就永不重发。
     fired_for      DATETIME,
+    -- 权限档位（plan / workspace / full）。事件触发时新建会话 → 用此档位定死；
+    -- 复用已有会话 → 取「会话现档 与 事件档」中更严那个（ModeAtMost），
+    -- 不允许通过重复事件提权（plan 档 Agent 建的日程触发时拿 workspace 就绕开了 plan）。
+    permission_mode TEXT NOT NULL DEFAULT 'workspace',
     created_at     DATETIME DEFAULT (strftime('%Y-%m-%d %H:%M:%f','now')),
     updated_at     DATETIME DEFAULT (strftime('%Y-%m-%d %H:%M:%f','now')),