feat(remote): remote workspaces — a window that is one machine

Split the framework-free half of tty7 into `tty7-core` and add a headless
`tty7-server` built on it, so a workspace's filesystem, git and session state
can live on another machine while the GUI stays where it is.

- `crates/tty7-core`: wire protocol, session daemon, PTY, native SSH engine and
  the domain model, with no gpui dependency. Module paths are unchanged.
- `crates/tty7-server`: the same daemon with no GUI attached, linked fully
  static against musl and pushed onto the remote box. One dependency, on
  purpose — a second one the GUI also needs belongs in core.
- `Host` trait + `HostId`/`HostRegistry`: every fs/git/watch call a workspace
  makes goes through the machine it belongs to. `LocalHost` answers on this
  box, `RemoteHost` over a routed control connection.
- `ui::host_ops`: the GUI's single door to a `Host`. Host calls block, so all
  of them run on the background executor with the result landed on the UI
  thread; de-duplication, staleness and error reporting live here rather than
  at each call site. Enforced by a CI grep.
- Connect flow: home page → pick a configured SSH host → the machine's own
  workspace list → a window bound to one workspace on it. Workspace switcher
  groups by machine, this computer included.
- CI: static musl builds of `tty7-server` for x86_64/aarch64 via
  cargo-zigbuild, a host-boundary grep, and version stamping factored out of
  the nightly workflow. Both new jobs are non-required so branch protection
  does not wedge open PRs.

Design and the interface contract it was built to are in
`docs/2026-07-27-remote-workspace-{design,impl-contract}.md`.

.github/scripts/bundle-linux.sh

@@ -13,7 +13,14 @@ set -euo pipefail
 
 TARGET="$1"
 ARCH="$2"
-VERSION="$(grep -m1 '^version' Cargo.toml | sed -E 's/.*"([^"]+)".*/\1/')"
+# Anchored on `= "` — see the note in bundle-macos.sh: the root manifest leads
+# with `version.workspace = true`, which a bare `^version` match would return
+# verbatim as the "version" and bake into every asset filename.
+VERSION="$(grep -m1 '^version = "' Cargo.toml | sed -E 's/.*"([^"]+)".*/\1/')"
+if [[ ! "$VERSION" =~ ^[0-9]+\.[0-9]+\.[0-9]+ ]]; then
+  echo "bundle-linux: could not read a version from Cargo.toml (got '$VERSION')" >&2
+  exit 1
+fi
 NAME="tty7-${VERSION}-linux-${ARCH}"
 STAGE="dist/${NAME}"