Add `title` field to SKILL.md frontmatter to set meaningful session names in the UI

When invoking a skill (e.g. /proceed), the Claude Code UI titles the session "Claude agent session initialization" regardless of which skill was used. There's currently no way to override this from within the skill.

Proposed change: Add an optional title field to SKILL.md frontmatter. When set, Claude Code would use it as the session title instead of the default.

\\\yaml
---
name: proceed
title: "Roadmap Execution — /proceed"
description: >
Reads the up-next file, proposes work for approval, executes, and writes the next up-next file.
---
\\\

Why this matters: Users with multiple skills in a project (e.g. /proceed, /research, /ingest) can't distinguish sessions at a glance in the session history. A static title per skill would be a minimal, low-friction fix — dynamic titling based on runtime context would be even better (e.g. using the task name from up-next.md).