feat(logic): establish derived-activity engine and agent-session protocols

Today we reached a major architectural conclusion: project history shouldn't be stored, it should be derived. We rejected the overhead of a separate SQLite event store in favor of an O(N) snapshot-diffing engine that computes human-readable narratives directly from the issues.jsonl source of truth.

Key Triumphs:
- Implemented O(N) diffing algorithm in src/lib/snapshot-differ.ts that transforms raw JSONL into 16 distinct social event types.
- Engineered a file-based persistence layer (src/lib/activity-persistence.ts) to solve the 'Next.js HMR Wiped My Memory' bug, ensuring project heartbeat survives server restarts.
- Developed the agent-session data model that unifies Beads, Activity, and Cross-Agent Mail into a single 'Mission' context.

Raw Honest Moment:
We struggled for over an hour with 'missing history' before realizing that development-mode reloads were purging our in-memory buffers. The shift to a file-backed ring buffer was a reactive pivot that became a core project strength.

src/lib/activity.ts

@@ -0,0 +1,75 @@
+import type { BeadIssueWithProject } from './types';
+
+/**
+ * 16 transition types for timeline activity events,
+ * as required by the bb-xhm.1 event model specification.
+ */
+export type ActivityEventKind =
+  | 'created'
+  | 'closed'
+  | 'reopened'
+  | 'status_changed'
+  | 'priority_changed'
+  | 'assignee_changed'
+  | 'type_changed'
+  | 'title_changed'
+  | 'description_changed'
+  | 'labels_changed'
+  | 'dependency_added'
+  | 'dependency_removed'
+  | 'comment_added'
+  | 'due_date_changed'
+  | 'estimate_changed'
+  | 'field_changed';
+
+/**
+ * Represents a discrete change or action derived from bead snapshots or interactions.
+ */
+export interface ActivityEvent {
+  /** Unique identity for the event instance (likely UUID) */
+  id: string;
+  
+  /** The type of transition that occurred */
+  kind: ActivityEventKind;
+  
+  /** The issue this event belongs to */
+  beadId: string;
+  
+  /** Display title of the issue at the time of the event */
+  beadTitle: string;
+  
+  /** The project key this issue belongs to */
+  projectId: string;
+  
+  /** Human-readable project name */
+  projectName: string;
+  
+  /** ISO8601 timestamp of when the event was detected or recorded */
+  timestamp: string;
+  
+  /** The actor who performed the action (assignee, owner, or session ID) */
+  actor: string | null;
+  
+  /** Data payload describing the change */
+  payload: {
+    /** The specific field name that changed (for property updates) */
+    field?: string;
+    
+    /** The previous value before the transition */
+    from?: any;
+    
+    /** The new value after the transition */
+    to?: any;
+    
+    /** Optional context message (e.g. comment text or close reason) */
+    message?: string;
+  };
+}
+
+/**
+ * A pair of snapshots used by the diffing engine to derive ActivityEvents.
+ */
+export interface SnapshotDiff {
+  previous: BeadIssueWithProject | null;
+  current: BeadIssueWithProject;
+}