Look around the machine before deciding to talk about it

The terminal and the canvas both needed a Chat, so they were missing from the
one screen where you are choosing which machine to work on. A draft is the
smallest thing that fixes it: an id, and the three facts behind it.

The trick is that a draft resolves to a *transient* Chat -- constructed, never
added to a session. `canvas.agent_ready`, `_executor`, `_load_agent`, `_save_agent`
and `agent_session.resolve` read exactly four attributes between them and none
of them queries or writes the row, so all of it works unchanged and nothing had
to learn what a draft is. Proven against a real sshd rather than a stub: a
transient chat opens and saves a project file over the same SFTP path a real one
uses, and the database stays empty throughout.

Chats are still created lazily. A draft is not a chat and never becomes one;
when the first prompt makes the real one, the shell is re-keyed into it and the
open tabs are copied across. `terminal.rekey` moves the registry key *and*
`session.chat_id`, because close_for_profile, close_for_owner and the reaper all
pop by the field -- a stale one would leave a dead session that `get` keeps
handing out. The shell is only adopted when its profile and directory match the
chat as finally resolved, since `_new_chat` settles an empty directory to the
connection's own; otherwise it is left alone rather than transplanted onto a
chat that says it runs elsewhere.

Two canvas sources are refused on a draft, by name, and one of them is a hole
rather than an inconvenience. `_load_file` authorises with
`attachment.chat_id != chat.id`, and an upload made on the new-chat screen is
stored with `chat_id=None` -- so a draft whose chat carried no id would make that
comparison `None != None`, which is False, and open every unclaimed attachment
its owner has. `as_chat` does set an id, so it already fails; the refusal is
stated anyway, because a guarantee that lives in an id-shaped coincidence is one
the next change breaks without noticing.

Adoption needed almost no JavaScript: start_chat already answers with
HX-Redirect, so the page reloads and the canvas adopts by construction while the
terminal reconnects to the re-keyed session and replays its scrollback -- the "a
reload is indistinguishable from a second tab" property working for us. What
re-points them mid-screen is a `lembas:agent-target` event, dispatched from
`setDir` and the connection select because assigning to a hidden field's value
fires nothing on its own. Driven under a DOM stub before committing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/services/agent/terminal.py

@@ -620,6 +620,30 @@ async def close_chat(chat_id: str, reason: str = CLOSED_REVOKED) -> bool:
     return True
 
 
+def rekey(old: str, new: str) -> Session | None:
+    """Move a live session from one id to another, keeping the shell.
+
+    What adoption is made of: a shell opened on the new-chat screen under a
+    draft id becomes the shell of the chat that screen turned into, with its
+    scrollback and whatever is half-typed at its prompt. Nothing reconnects --
+    the browser navigates after `start_chat` and attaches to the session now
+    living under the real id, which is the "a reload is indistinguishable from a
+    second tab" property working for us rather than against us.
+
+    **Both the key and the field.** `close_for_profile`, `close_for_owner` and
+    the reaper all pop by `session.chat_id` rather than by the key they found it
+    under, so a stale field would leave a closed session in the registry that
+    `get` keeps handing out and `count_for` keeps counting.
+    """
+    session = _SESSIONS.pop(old, None)
+    if session is None:
+        return None
+    session.chat_id = new
+    _SESSIONS[new] = session
+    log.info("terminal adopted %s -> %s", old, new)
+    return session
+
+
 async def close_for_profile(profile_id: str) -> int:
     """End every shell opened on one connection.