feat(protocol): thread/list, thread/fork and thread/archive - #231

The sidebar archived and deleted session files through Tauri while the
app-server served the same threads. That is not only a second reader — the
app-server is the single owner and writer of thread storage, so a renderer
deleting files behind it can pull the ground out from under an open writer or
leave the index pointing at something gone.

Three call sites, not the two THREE_WAY_REVIEW recorded. `window.deepcode.
sessions.list()` has preferred the protocol since #231, but `Sidebar.tsx`
bypassed the shim and called `listSessions()` directly — so the list was a
second reader too, with its own row shape and its own sort.

`thread/delete` is new. The protocol could list, fork and archive but not
delete, which is why delete had nowhere to go. It sits under the existing
`threadManagement` capability and mirrors `archive`: 404 on a thread that does
not exist rather than silently succeeding, and a store that cannot delete says
so instead of quietly archiving instead — being helpful about a destructive verb
by doing a different one is the worst available answer.

Deleting removes both representations. The protocol snapshot and the canonical
session projection share an id and are two views of one thing, and the composite
`list` reads both; removing one left the row reappearing on the next refresh as
an empty session that could not be opened. `SessionManager.delete` takes the
stream, the legacy stream, the meta sidecar, the writer lock and the per-session
directory — the listing reads the sidecar, so leaving it behind is not a tidy
half-delete.

The Tauri commands stay as the fallback for a sidecar too old to serve the
methods, matching what #231 established for `list`.

Co-Authored-By: Claude Opus 5 <[email protected]>

The sidebar archived and deleted session files through Tauri while the
app-server served the same threads. That is not only a second reader — the
app-server is the single owner and writer of thread storage, so a renderer
deleting files behind it can pull the ground out from under an open writer or
leave the index pointing at something gone.

Three call sites, not the two THREE_WAY_REVIEW recorded. `window.deepcode.
sessions.list()` has preferred the protocol since #231, but `Sidebar.tsx`
bypassed the shim and called `listSessions()` directly — so the list was a
second reader too, with its own row shape and its own sort.

`thread/delete` is new. The protocol could list, fork and archive but not
delete, which is why delete had nowhere to go. It sits under the existing
`threadManagement` capability and mirrors `archive`: 404 on a thread that does
not exist rather than silently succeeding, and a store that cannot delete says
so instead of quietly archiving instead — being helpful about a destructive verb
by doing a different one is the worst available answer.

Deleting removes both representations. The protocol snapshot and the canonical
session projection share an id and are two views of one thing, and the composite
`list` reads both; removing one left the row reappearing on the next refresh as
an empty session that could not be opened. `SessionManager.delete` takes the
stream, the legacy stream, the meta sidecar, the writer lock and the per-session
directory — the listing reads the sidecar, so leaving it behind is not a tidy
half-delete.

The Tauri commands stay as the fallback for a sidecar too old to serve the
methods, matching what #231 established for `list`.

Co-Authored-By: Claude Opus 5 <[email protected]>