On Discord, Telegram, Slack and Matrix a plain `/branch` used to rebind the
CURRENT chat/thread's session key to the clone, ending the original session
on that surface. The user could not keep the original path live while
exploring an alternate one — the opposite of what a branch is for.
Now the handler opens a sibling thread through the adapter's existing
`create_handoff_thread` BEFORE cloning (a failed create never orphans a
branch row), binds the thread's own session key to the clone with the
thread's routing columns written at create time, and leaves the origin key
untouched. `/branch --here` keeps the legacy in-place switch; platforms
without threads, DMs, unknown Discord parents and adapters that cannot open
a thread fall back to in-place with a one-line note. The CLI strips the
flag through the same parser so `--here` never becomes a session title.
Destination source shapes mirror each adapter's inbound key (Discord keys
threads on their own id; Telegram/Slack/Matrix on the parent chat), the
same rules the CLI->platform handoff uses.
Live repro (real gateway + real Slack adapter against a stand-in Slack
Socket Mode/Web API): base ends the origin session and rebinds its key;
fixed posts the thread seed, replies "this chat stays on it", the origin
thread keeps its session and the follow-up typed in the new thread lands on
the branch (parent_session_id = origin).
Design and first implementation by Angello Picasso (#66014, #66024);
this is a slim port onto the split slash_commands_* layout.
Co-authored-by: Angello Picasso <angello.picasso@devsu.com>
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