Support `/rename` in VS Code agent sessions, and let the agent invoke it

Problem

/rename (added in #1776) works in the terminal CLI, but not for Copilot CLI sessions hosted in VS Code's Agents window:

• Typing /rename in an agent session's chat has no effect there — renaming is only available through the VS Code UI (verified 2026-07-23).
• There is no way for the agent itself to rename its session — no tool, and no slash-command it can run.

Proposal

1. Honor /rename NEW_NAME in VS Code agent sessions, matching the terminal CLI behavior.
2. Expose rename to the agent (a tool, or agent-invocable /rename), so workflows can set titles programmatically.

Use case

Session-hygiene rituals: I keep many long-running agent sessions and prefix titles with a status word as threads settle — DONE …, PARKED …, CONT'D … — so the session picker doubles as a work board. Today every close-out ends with manual rename homework; an agent-invocable rename would let the close-out workflow set the title itself (and lower-stakes: agents could title sessions descriptively from the start, cf. #3365 on auto-naming).

Notes from poking at the internals

• A UI rename syncs into ~/.copilot/session-state/UUID/workspace.yaml (name: + user_named:), so agents can already read current titles (verified 2026-07-24).
• Writing workspace.yaml directly doesn't work as a workaround: VS Code caches session labels elsewhere (workspace state.vscdb, agentSessions.model.cache) and doesn't notice the change.

Related

• #1776 — added /rename (terminal CLI)
• #3365 — auto-generated session names not working
• #3415 — show session name at all times
• #2966 — broader session-management tooling

Start by tracing the existing terminal /rename implementation from #1776, then inspect workspace.yaml and the VS Code state.vscdb/agentSessions.model.cache behavior described here. Done means /rename NEW_NAME works in VS Code agent chat and an agent has a supported way to rename its session.