sl0p: carry pane and tab purposes over the wire

`new-tab --layout` / `--layout-string` created the tab and silently dropped
every purpose the layout declared -- the tab's own (which rides the tab's root
layout) and each pane's. Layouts applied at session creation kept theirs, so
this only broke tabs added at runtime, which is exactly what an orchestrator
does. Found by integration-testing the sl0ppi CLI against a real session rather
than trusting that the documented feature worked on this path.

Cause: TiledPaneLayout and FloatingPaneLayout had no protobuf `purpose` field,
so anything travelling as an Action lost it. The original workspace-hints commit
says as much in a comment -- "adding a proto field would force regenerating the
committed assets" -- because generated prost output is checked in and protoc was
not available. It is now (a user-local protoc 25.1, no sudo, no system package),
so the deferral can be paid off properly instead of worked around.

There are two protobuf schemas and the distinction matters: plugin_api/ is what
plugins speak, client_server_contract/ is what the CLI speaks. The first attempt
fixed only plugin_api and changed nothing observable; both now carry the field.

  plugin_api/action.proto             TiledPaneLayout.purpose = 14
                                      FloatingPaneLayout.purpose = 13
  client_server_contract/common_types TiledPaneLayout.purpose = 16
                                      FloatingPaneLayout.purpose = 15

Purposes are sanitised on ingest, so a value arriving over IPC obeys the same
invariant as one parsed from a layout file (bounded token, <=64 chars) whoever
constructed the message. Verified live: a tab declaring `rm -rf /; echo "x"` is
stored as `rm-rf/echox`, and a 200-char pane purpose arrives capped at 64.

Tests: two new wire round-trips (a NewTab layout carrying tab, pane and floating
purposes; and the sanitiser). zellij-utils 473 passed, zellij-server 1499
passed. Live: `new-tab --layout-string` now reports
purpose='project:added/nested' with panes 'agent:main' and 'logs:app'.

0 files changed, 0 insertions, 0 deletions