[FEATURE] Auto-group sidebar sessions by title prefix (rule-based, with default-collapsed groups)

Problem

I run a multi-session workflow (the "conductor method") where sessions fall into a few fixed classes by title prefix:

• Orchestrator: ... — the ones I actively follow
• Conduct: ... — worker sessions I rarely open unless something needs me
• Charter: ... — planning sessions
• everything else — one-off chats

Today the only way to keep these organized is to manually right-click → move each new session into a group. With sessions spawning constantly, they immediately pile into "Ungrouped" and I lose the separation. I have to re-sort by hand every time.

Related but not the same: #82788 (auto-assign by parent), #91660 (MCP tool to assign), #76472 (composable naming). None lets me say "titles starting with X go in group Y automatically."

Request

Rule-based auto-grouping of sidebar sessions by title prefix (or a simple pattern). Something like:

• A small config (settings.json or a UI panel) mapping a prefix → a group name
• New sessions whose title matches a rule are placed in that group automatically, no manual move
• Sessions matching no rule stay in "Ungrouped"
• Each group can be default-collapsed, so classes I don't watch (e.g. Conduct:) stay folded away while the group I follow (Orchestrator:) stays visible

Example config sketch:

"sessionGroups": {
  "rules": [
    { "prefix": "Orchestrator:", "group": "Orchestrators", "collapsed": false },
    { "prefix": "Conduct:",      "group": "Conductors",    "collapsed": true }
  ]
}

Why title prefix specifically

Parent-based grouping (#82788) doesn't cover it — orchestrator and conductor sessions aren't always in a parent/child relationship, and I want the class (which I already encode in the title) to drive placement, not the spawn tree. The MCP-tool approach (#91660) works per-session but still needs an agent to run it each time; a declarative prefix rule is set-once.

Nice-to-have

• Pattern/regex, not just literal prefix
• Rules re-evaluated on rename, so retitling a session re-homes it