Tool timeline: skip cd/pushd prefix in shell command labels

## Problem

When the agent runs shell commands with a `cd` prefix (very common — agents typically change to the project directory before running the actual command), the tool timeline shows the uninformative directory change instead of the meaningful command:

```
[tool]> Running: cd C:\Tools\my-project
[tool][OK 5.0s] Running: cd C:\Tools\my-project
[tool]> Running: cd C:\Tools\my-project
[tool][OK 9.6s] Running: cd C:\Tools\my-project
```

The user sees a wall of identical `cd` lines with no indication of what the agent is actually doing (creating issues, running tests, editing configs, etc.). The tool timeline is the primary way users track agent progress, and this makes it nearly useless for chained commands.

## Typical command patterns affected

```bash
cd /repo && gh issue create --title "..."
cd C:\Tools\project && npx tsc --noEmit
cd /workspace && npm run test
pushd /tmp && npm run build
Set-Location C:\src && git status
```

## Proposed solution

When rendering shell tool execution labels, split the command string on chain operators (`&&`, `||`, `;`) and skip segments that are pure directory changes (`cd`, `pushd`, `Set-Location`). Show the first meaningful command segment instead.

**Before**: `Running: cd C:\Tools\my-project`
**After**: `Running: gh issue create --title "..."`

If the entire command is just a `cd`, show it as-is (it is the only thing happening).

## Minimal logic (what we implemented server-side for our own UI)

```typescript
const segments = cmd.split(/\s*(?:&&|\|\||;)\s*/);
const meaningful = segments.find(
(s) => !/^\s*(?:cd|pushd|Set-Location)\s/i.test(s),
);
const label = (meaningful ?? segments[0] ?? "").trim();
```

This is a small UX improvement with outsized impact on readability when the agent chains commands.