feat: add audit/activity log view with timeline and event filtering (#17)

Add a dedicated /audit route displaying a chronological timeline of
orchestration events (state transitions, tool invocations, errors,
messages) grouped by session. Includes session selector, event type
filtering, URL deep-linking via ?session=id, and audit event capture
during chat streaming.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

implementation-plans/issue-017.md

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+# Issue #17 — Audit/Activity Log View
+
+**Status: COMPLETED**
+
+## Overview
+
+Timeline view of all orchestration events for a session — state transitions, tool invocations, errors. Populated from the streamed responses captured during the session.
+
+## Implementation Steps
+
+### 1. Audit Event Store (`src/lib/stores/audit.svelte.ts`) — COMPLETED
+
+- Created `AuditEvent` type with `id`, `sessionId`, `timestamp`, `eventType`, `details`, `state?`
+- Event types: `state_change`, `tool_invocation`, `error`, `message`
+- Store events grouped by session using `SvelteMap`, persisted to `localStorage`
+- Functions: `addEvent()`, `getEventsBySession()`, `getAllSessions()`, `clearSession()`
+- Sample data seeded for demo sessions
+
+### 2. AuditTimeline Component (`src/lib/components/AuditTimeline.svelte`) — COMPLETED
+
+- Vertical timeline with left-aligned time markers and event cards
+- Color-coded type badges: state change=blue, tool=green, error=red, message=gray
+- Each event shows timestamp, type badge, state label (when applicable), and detail text
+- Date headers when events span multiple days
+- Hover shadow effect on event cards
+
+### 3. Audit Route (`src/routes/audit/+page.svelte`) — COMPLETED
+
+- Dedicated `/audit` route with session selector dropdown and timeline
+- Filter controls for event type (all, state changes, tool invocations, errors, messages)
+- URL query param `?session=id` for deep linking to specific session
+- Empty states when no events or no session selected
+- Header with back-to-chat link and "Sample Data" badge
+
+### 4. Audit Route Config (`src/routes/audit/+page.ts`) — COMPLETED
+
+- `export const prerender = false` and `export const ssr = false`
+
+### 5. Homepage Navigation (`src/routes/+page.svelte`) — COMPLETED
+
+- Added "Audit Log" navigation link alongside existing Chat, Lineage, Memory links
+
+### 6. Chat Page Integration (`src/routes/chat/+page.svelte`) — COMPLETED
+
+- Added "Audit" link in the header navigation bar
+- Capture orchestration state changes during streaming into the audit store
+- Capture errors into the audit store
+
+## Files Changed
+
+- `src/lib/stores/audit.svelte.ts` (new)
+- `src/lib/components/AuditTimeline.svelte` (new)
+- `src/routes/audit/+page.svelte` (new)
+- `src/routes/audit/+page.ts` (new)
+- `src/routes/+page.svelte` (modified)
+- `src/routes/chat/+page.svelte` (modified)