update: roll back what the restart actually deploys

On the deployment deploy/README.md documents, source_dir and install_dir are
the same tree and the restart command rebuilds the image from it. The
Dockerfile builds from cmd/ and internal/ and .dockerignore keeps the host
binaries out, so restoring the snapshotted binaries restored bytes nothing
reads. A bad commit therefore cost two health timeouts and two image builds
and ended in ErrRollbackFailed with an instruction to copy files back by hand,
which would not have helped either.

A deployment that rebuilds from source now has to say how the source is put
back. source_rollback "git" records the commit before the update and checks it
back out before the rollback restart. It refuses a dirty tree, because the
recorded commit does not describe one and a forced checkout would delete his
work. A build-from-source config that says nothing is refused by Validate, at
startup, rather than at the one rollback that mattered.

Also in this change, all from the same review:

  - MethodPing, the one method a locked daemon answers. Preflight passed on an
    unlocked daemon and the post-restart Presence read failed on a locked one,
    so a good update read as SHE IS PROBABLY DOWN once the env key is gone.
  - A dial failure is reported apart from a read failure. The documented
    socket is under /var/lib/docker, which a non-root operator cannot
    traverse, and "she is not answering" was the wrong diagnosis.
  - Verify refuses to run as root over a tree owned by someone else. It runs
    make build and make test in place, and root-owned artifacts break his next
    ordinary make.
  - A rollback no longer reverts config_files. That undid every config edit
    since the last apply, phraser.model_path among them.
  - The verify-failure path no longer reports rolled_back for a compile error.
  - waitHealthy caps each attempt at the remaining budget, so a 90s timeout
    cannot run to 99s.
  - tail cuts on a rune boundary. Russian test names showed the seam.
  - The claim that mavend does not import internal/update is replaced with
    what is enforced: mavend constructs no Updater and nothing can call Apply.
  - snapshot_dir inside source_dir is refused. It landed in the build context.

Found in review of #69.

internal/ipc/client.go

@@ -75,6 +75,7 @@ var readOnlyMethods = map[Method]bool{
 	MethodMCPServers:           true,
 	MethodDayPlan:              true,
 	MethodRecentEvents:         true,
+	MethodPing:                 true,
 }
 
 // Dial connects to a core socket at path and returns a Client. The module
@@ -636,5 +637,16 @@ func (c *Client) RevertFact(ctx context.Context, key string) (int64, error) {
 	return result.NewID, nil
 }
 
+// Ping asks whether the daemon is there, and whether it is locked. It is not a
+// CoreAPI method: it touches no store, so it answers before the passkey
+// assertion that every other read waits for.
+func (c *Client) Ping(ctx context.Context) (PingResp, error) {
+	var r PingResp
+	if err := c.call(ctx, MethodPing, nil, &r); err != nil {
+		return PingResp{}, err
+	}
+	return r, nil
+}
+
 // Compile-time check: *Client satisfies CoreAPI.
 var _ CoreAPI = (*Client)(nil)