feat(event,storage): add events table substrate for OSS redesign

New internal/event package defines the Event type with canonical Kind
and Actor constants. New events table is appended via additive migration
(id, task_id, seq, ts, kind, actor, payload_json) with seq as per-task
monotonic, assigned by the storage layer inside a transaction.

CreateEvent + ListEvents support the future migration of question_json,
interactions_json, summary, and elaboration_input off the tasks table
into a unified, replayable event stream. No existing code paths are
modified yet; this is pure substrate for Phase 1.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

1 files changed, 50 insertions, 0 deletions

diff --git a/internal/event/event.go b/internal/event/event.go
new file mode 100644
index 0000000..2573fde
--- /dev/null
+++ b/internal/event/event.go
@@ -0,0 +1,50 @@
+// Package event defines the Event type and canonical kinds/actors for the
+// task observability stream. Events are immutable, per-task, monotonically
+// sequenced records persisted by internal/storage.
+package event
+
+import (
+	"encoding/json"
+	"time"
+)
+
+// Kind enumerates the canonical event types.
+type Kind string
+
+const (
+	KindStateChange          Kind = "state_change"
+	KindAgentMessage         Kind = "agent_message"
+	KindClarificationRequest Kind = "clarification_request"
+	KindClarificationAnswer  Kind = "clarification_answer"
+	KindHumanMessage         Kind = "human_message"
+	KindSummary              Kind = "summary"
+	KindCommitMade           Kind = "commit_made"
+	KindSubtaskSpawned       Kind = "subtask_spawned"
+	KindCostReport           Kind = "cost_report"
+	KindExecutionStarted     Kind = "execution_started"
+	KindExecutionEnded       Kind = "execution_ended"
+)
+
+// Actor identifies the originator of an event.
+type Actor string
+
+const (
+	ActorAgent   Actor = "agent"
+	ActorUser    Actor = "user"
+	ActorChatbot Actor = "chatbot"
+	ActorSystem  Actor = "system"
+	ActorWebhook Actor = "webhook"
+)
+
+// Event is a single timestamped record on a task's audit/observability stream.
+// Once written, events are append-only — the seq column is per-task monotonic
+// and assigned by the storage layer on insert.
+type Event struct {
+	ID        string          `json:"id"`
+	TaskID    string          `json:"task_id"`
+	Seq       int64           `json:"seq"`
+	Timestamp time.Time       `json:"timestamp"`
+	Kind      Kind            `json:"kind"`
+	Actor     Actor           `json:"actor"`
+	Payload   json.RawMessage `json:"payload"`
+}