sidebar activity timeline replaces tool display in chat

- New ActivityTimeline: unified timeline in sidebar showing all agent actions
  (goal, plan steps, tool calls, knowledge, completion) in reverse chron order
- activityLog derived store merges messages + planSteps + currentTask
- AgentIndicator stays in chat (thinking/working indicator), simplified props
- ToolCallGroup removed from chat — tools visible only in sidebar timeline
- SessionDigest replaced by ActivityTimeline
- PlanProgress restored in sidebar (conceptual steps, separate from timeline)

plans/2026-07-14-activity-timeline.md

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+# 2026-07-14 — Unified sidebar activity timeline
+
+**Status:** Planned
+
+## Current state (broken)
+
+The sidebar has three sections that appear/disappear independently:
+
+| Section | When visible | Shows |
+|---|---|---|
+| Plan (top) | When `$planSteps.length > 0` | Plan step names with progress bar |
+| Tool activity (middle) | When `toolCount > 0` | Compact tool list, grouped by turn |
+| This session (bottom) | When `digest.total_executions > 0` | Post-hoc execution count + knowledge |
+
+State changes cause sections to **pop in/out** as the agent moves between
+planning → executing → done. The "0 tools · 0 running" counter flashes
+briefly then vanishes. Tool activity appears/disappears between turns.
+
+## Target: single unified timeline
+
+One section, always present when a session is loaded. Every agent action
+appears as an entry in reverse-chronological order (newest at top).
+
+```
+┌─ Activity ───────────────────────── ─┐
+│                                       │
+│ ✓ Task completed: "Upgraded 4 LXCs"   │  ← newest
+│ ◉ Running: apt upgrade on lxc:dns     │
+│ ✓ run: apt upgrade on lxc:gitea       │  ← tool completed
+│ ✓ Verified gitea: HTTP 200            │
+│ ◉ Step 3/5 — Upgrade dns             │  ← plan step running
+│ ✓ Step 2/5 — Upgrade gitea           │  ← plan step done
+│ ✓ run: apt upgrade on lxc:nfs-export  │
+│ ◉ Step 1/5 — Upgrade nfs-export      │
+│ ✓ Knowledge recorded                 │
+│ 📋 Plan set: 5 steps                 │  ← plan proposed
+│ 🎯 Goal: Upgrade 4 low-risk LXCs      │  ← goal set
+│                                       │  ← oldest
+└───────────────────────────────────────┘
+```
+
+### Entry types
+
+| Type | Icon | Example description |
+|---|---|---|
+| `goal` | 🎯 | "Audit all LXCs for updates" |
+| `plan` | 📋 | "Plan set: 5 steps" |
+| `step_start` | ◉ spinner | "Step 2/5 — Upgrade gitea" |
+| `step_done` | ✓ | "Step 2/5 — Upgrade gitea" |
+| `tool_start` | ◉ spinner | "run: Upgrade nfs-export (21 pkgs)" |
+| `tool_done` | ✓ | "run: 0 upgraded, 0 newly installed" |
+| `tool_error` | ✗ | "run: SSH handshake failed" |
+| `knowledge` | ✨ | "Recorded: How to run fleet upgrades" |
+| `complete` | ✓ | "Task completed: success" |
+| `question` | ❓ | "Asked: Which host for the LXC?" |
+| `error` | ✗ | "Auto-resume failed: context deadline exceeded" |
+
+### Data source
+
+Entries come from all available sources, merged and deduplicated:
+1. **`toolTimeline` store** (live tool_use/tool_result pairs)
+2. **`planSteps` store** (step status transitions)
+3. **Session digest API** (knowledge created, final outcome)
+4. **`currentTask` store** (goal, status)
+
+Deduplication: when a plan step links to a tool call via `execution_id`, show
+them as one entry instead of two (e.g. "Step 3: Upgrade dns ◉ running" includes
+the tool — don't show a separate "run: apt upgrade" entry).
+
+### Behavior
+
+- **Always visible** when `$currentSession` is set
+- **Reverse chronological** — newest entries at top, scrolls naturally
+- **Auto-expands** the entry for the currently-running tool/step
+- **Collapses** completed entries to one line (expandable)
+- **Polls** every 3s for live updates (same as current startPolling)
+- **No flashing** — entries only change status in place (tool_start → tool_done), never removed
+- **Persists** across page navigation (rehydrated from REST on load)
+- **Empty state** when no session: "Open a session to see agent activity"
+
+### What gets removed from chat
+
+- **ToolCallGroup** — the compact tool counter. Tools live in the timeline now.
+- **AgentIndicator at bottom** — partially. Keep it ONLY for the initial
+  "thinking" state (before any tools fire). Once the first tool fires, the
+  timeline is the source of truth and the chat indicator is redundant.
+  Actually: remove it entirely. The timeline IS the indicator.
+
+### What stays in chat
+
+- **Agent text responses** — the thinking, conclusions, reports
+- **InlineApproval cards** — approvals need operator action, must be in chat
+- **Inline tool renderers** — entity cards, health summary, etc. (informational)
+- **User messages** — obviously
+
+## Implementation
+
+### 1. Data layer: `activityLog` derived store
+
+Add to `chat.ts`:
+
+```ts
+export interface ActivityEntry {
+  id: string
+  type: 'goal' | 'plan' | 'step_start' | 'step_done' | 'step_failed' |
+        'tool_start' | 'tool_done' | 'tool_error' |
+        'knowledge' | 'complete' | 'question' | 'error'
+  description: string
+  detail?: string       // tool result text, step detail, etc.
+  timestamp: number      // Date.now() when created
+  seq?: number           // plan step seq, for ordering
+  toolName?: string      // for tool entries
+  status: 'running' | 'done' | 'failed'
+  collapsed: boolean     // initial collapsed state (true for completed)
+}
+```
+
+Derived reactively from `messages`, `planSteps`, `currentTask`, and session
+digest data. Uses `$derived.by()` to recompute when any source changes.
+
+### 2. New component: `ActivityTimeline.svelte`
+
+Replaces all three sidebar sections. Renders `activityLog` entries as a
+vertical timeline with connecting lines.
+
+### 3. Remove from chat
+
+- `<ToolCallGroup>` rendered in chat
+- `<AgentIndicator>` at bottom
+
+### 4. Update TaskContextPanel
+
+Replace PlanProgress + SessionDigest with ActivityTimeline.