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/cmd/server/main.go

@@ -182,6 +182,8 @@ func main() {
 			// 本任务的往返预算：在对话页里随时可改
 			r.Get("/sessions/{id}/budget", handler.GetSessionBudgetHandler)
 			r.Put("/sessions/{id}/budget", handler.UpdateSessionBudget)
+			// 本任务的权限档位：在对话页里随时可改
+			r.Put("/sessions/{id}/permission", handler.UpdateSessionPermission)
 			// Agent 在正文里提的改名建议：读取与驳回（接受走上面的 PUT alias）
 			r.Get("/sessions/{id}/rename-proposal", handler.GetRenameProposal)
 			r.Post("/sessions/{id}/rename-proposal/dismiss", handler.DismissRenameProposal)