step 5 (partial): strip dropped packages from production code

Removed all references to dropped packages in non-test code.
Zero dropped imports, store refs, or model types remaining.

Major removals:
  - scheduler/ package (entire dir) — tasks moved to extension track
  - taskutil/ package (entire dir)
  - health/ package (entire dir) — provider/tool health
  - sandbox/provider_module.go — provider.complete module
  - handlers: workflow_entry, workflow_instances, workflow_forms,
    workflow_assignments, workflow_monitor, health_admin (6 files)
  - auth/session.go, middleware/session_auth.go
  - store/{postgres,sqlite}/sessions.go, tasks.go

Rewrites:
  - store/{postgres,sqlite}/health.go — kernel-only Prune
    (ws_tickets, rate_limit_counters, stale presence)
  - handlers/admin.go: 847→467 lines (stripped provider CRUD)
  - handlers/teams.go: 520→411 lines (stripped model listing)
  - sandbox/runner.go: removed ProviderResolver
  - sandbox/workflow_module.go: 216→83 lines (definition-only)
  - main.go: 1579→1325 lines (stripped dropped package init)
  - pages/pages.go: stripped channel/session lookups
  - events/types.go: stripped chat/channel/workspace event routes
  - models: stripped stale types, constants, notification types

Store interface cleanup:
  - Removed SessionStore, TaskStore from Stores struct
  - Stripped workflow assignment methods from WorkflowStore iface
  - Stripped assignment methods from PG+SQLite workflow stores
  - Updated testhelper.go table list to kernel-only

Migrations: removed 008_tasks.sql (both dialects)

-9665/+69 lines across 51 files.

server/database/migrations/postgres/008_tasks.sql

@@ -1,69 +0,0 @@
--- ==========================================
--- Switchboard Core — 008 Tasks
--- ==========================================
--- Task scheduling and run history.
--- Stripped: persona_id (kept nullable, no FK), output_channel_id,
--- provider_config_id. Output modes: notification | webhook | log.
--- ==========================================
-
-CREATE TABLE IF NOT EXISTS tasks (
-    id                 UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-    owner_id           UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
-    team_id            UUID REFERENCES teams(id) ON DELETE SET NULL,
-    name               TEXT NOT NULL,
-    description        TEXT DEFAULT '',
-    scope              TEXT NOT NULL DEFAULT 'personal'
-                       CHECK (scope IN ('personal', 'team', 'global')),
-    task_type          TEXT NOT NULL DEFAULT 'action'
-                       CHECK (task_type IN ('action', 'workflow', 'system')),
-    system_function    TEXT DEFAULT '',
-    persona_id         TEXT,
-    model_id           TEXT,
-    system_prompt      TEXT DEFAULT '',
-    user_prompt        TEXT DEFAULT '',
-    workflow_id        UUID REFERENCES workflows(id) ON DELETE SET NULL,
-    tool_grants        JSONB,
-    schedule           TEXT NOT NULL,
-    timezone           TEXT NOT NULL DEFAULT 'UTC',
-    is_active          BOOLEAN NOT NULL DEFAULT true,
-    trigger_token      TEXT UNIQUE,
-    max_tokens         INTEGER NOT NULL DEFAULT 4096,
-    max_tool_calls     INTEGER NOT NULL DEFAULT 10,
-    max_wall_clock     INTEGER NOT NULL DEFAULT 300,
-    output_mode        TEXT NOT NULL DEFAULT 'log'
-                       CHECK (output_mode IN ('notification', 'webhook', 'log')),
-    webhook_url        TEXT,
-    webhook_secret     TEXT,
-    notify_on_complete BOOLEAN NOT NULL DEFAULT false,
-    notify_on_failure  BOOLEAN NOT NULL DEFAULT true,
-    last_run_at        TIMESTAMPTZ,
-    next_run_at        TIMESTAMPTZ,
-    run_count          INTEGER NOT NULL DEFAULT 0,
-    created_at         TIMESTAMPTZ NOT NULL DEFAULT now(),
-    updated_at         TIMESTAMPTZ NOT NULL DEFAULT now()
-);
-
-CREATE INDEX IF NOT EXISTS idx_tasks_next_run ON tasks(next_run_at) WHERE is_active = true;
-CREATE INDEX IF NOT EXISTS idx_tasks_owner ON tasks(owner_id);
-CREATE INDEX IF NOT EXISTS idx_tasks_team ON tasks(team_id);
-CREATE UNIQUE INDEX IF NOT EXISTS idx_tasks_trigger_token
-    ON tasks(trigger_token) WHERE trigger_token IS NOT NULL;
-
--- ── Task Runs ───────────────────────────────
-
-CREATE TABLE IF NOT EXISTS task_runs (
-    id               UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-    task_id          UUID NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
-    status           TEXT NOT NULL DEFAULT 'running'
-                     CHECK (status IN ('queued', 'running', 'completed', 'failed', 'budget_exceeded', 'cancelled')),
-    trigger_payload  TEXT,
-    started_at       TIMESTAMPTZ NOT NULL DEFAULT now(),
-    completed_at     TIMESTAMPTZ,
-    tokens_used      INTEGER DEFAULT 0,
-    tool_calls       INTEGER DEFAULT 0,
-    wall_clock       INTEGER DEFAULT 0,
-    error            TEXT
-);
-
-CREATE INDEX IF NOT EXISTS idx_task_runs_task ON task_runs(task_id);
-CREATE INDEX IF NOT EXISTS idx_task_runs_status ON task_runs(task_id, status);