[FEATURE] Expose a way to set the session/tab title in the VSCode extension (ungate /rename or add a command)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Running several Claude Code sessions in the VSCode extension at once (e.g. multiple PR reviews plus a separate task) leaves every editor tab titled similarly and hard to tell apart. There's no supported way to set a per-session tab/title in the native-UI/webview mode, so concurrent sessions aren't distinguishable at a glance.

Proposed Solution

Make session/tab titling reachable from the native UI. The plumbing already exists in the extension (2.1.170):

• The webview calls renameTab(title, …) → sends a rename_tab request → the host sets panelTab.title.
• There's a terminalTitleFromRename setting ("Whether /rename updates the terminal tab title").

But it isn't reachable in the webview/native-UI mode:

• /rename returns "isn't available in this environment" there (it appears gated to terminal mode).
• No claude-vscode.* command is contributed for tab/title/rename, so there's nothing to bind a key to or invoke from the command palette.

Either:

1. Ungate /rename in the native-UI/webview mode (it already drives rename_tab), or
2. Add a contributed command (e.g. claude-vscode.renameTab) and/or a setting, so the tab/session title can be set per session (and scripted/keybound).

Alternative Solutions

• A third-party extension can't do this — VSCode only lets a webview panel's owner set its title, and Tab.label (window.tabGroups) is read-only.
• Terminal mode (claudeCode.useTerminal: true) does let /rename/OSC set the tab title, but that gives up the native webview UI.

Feature Category

Configuration and settings

Use Case Example

1. Open multiple Claude Code sessions in VSCode for different work items.
2. Set each session's tab title to what it's tracking (e.g. a PR or ticket id).
3. Glance at the tab strip and immediately know which session is which, instead of hovering/guessing.

Additional Context

Environment: Claude Code VSCode extension 2.1.170, VSCode on macOS (Apple Silicon). The existing rename_tab IPC + terminalTitleFromRename setting suggest the capability is mostly built — the ask is to make it reachable from the native UI.