Feature request: choose the terminal title, and set it mid-session with a slash command (refiles #62384)

## Summary

Let the terminal title be chosen, and let that choice be changed mid-session
with a slash command rather than only at launch.

This re-raises #62384, which asked for a configurable terminal title and for
task status to move to OSC 9;4. That issue was closed NOT PLANNED by the
staleness bot with no human reply, so it is refiled here as the bot's closing
message invites, with one addition: a slash command.

## The problem

Claude Code writes an auto-generated conversation summary to the terminal
title. With several sessions open, the titles are a poor way to tell tabs
apart:

- the summary is generated from the conversation, so it does not identify the
workspace
- it can go stale. A session that has moved on to entirely different work can
still be advertising a title from an hour ago, which is worse than no title,
because it is confidently wrong
- suppressing the title in the terminal (`suppressApplicationTitle`) also
removes the task-completion indicator, since that appears to be encoded as a
title prefix rather than sent separately

I run three terminal tabs into the same host, all of them showing the same
connection name, and the title is the only thing that distinguishes them. A
stale or opaque summary makes that distinction useless exactly when several
sessions are running, which is the case it matters in.

## Requested

1. A choice of what goes in the title -- the project or working directory
name, the conversation summary, or a template combining them.

2. **A slash command to set it at any point in a session**, for example
`/title ` to pin a literal title and `/title auto` to return to the
generated one.

This is the part #62384 did not cover, and it is the part I would use most.
A launch flag or a settings file only helps if the right answer is known
before the session starts. In practice what a session is doing changes
every few turns, and the moment I want to relabel a tab is the moment I am
already inside it. Having to exit and relaunch to rename a tab defeats the
purpose.

3. Send task status over OSC 9;4, independently of the title, so choosing a
fixed title does not cost the completion indicator. This is #62384's second
point and still stands.

## Note

Points 1 and 3 are #62384's; point 2 is new. If the earlier issue is still
under consideration internally, treat this as a request to add the slash
command to it rather than as a duplicate.