fleet: put the bootstrapped Node on PANE_PATH (#1256)

On a host with no system Node, tools/install.sh bootstraps one into
~/.mosaic/node/current/bin and records it in ~/.profile. The fleet unit runs
`env -i ... bash --noprofile --norc`, so ~/.profile is never read — that is
deliberate — and _build_runtime_bin_prefix did not name the directory itself.
Runtime binaries are `#!/usr/bin/env node`, so the pane resolved `mosaic` and
then died on `env: 'node': No such file or directory` after an install that
reported success. Measured on a greenfield VM.

The existing `npm config get prefix` branch cannot cover it: that reports a
package prefix (~/.npm-global), never a Node runtime directory.

The test case asserts the property rather than the string — it runs the pane for
real with a Node-shebang `mosaic` and requires the pane to have executed. A PATH
substring check would pass on a fix that put the directory in the wrong position.
Red without the launcher change, green with it, rest of the suite unaffected.

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -268,6 +268,15 @@ esac
 _build_runtime_bin_prefix() {
   local candidates=()
   if [ -n "$MOSAIC_RUNTIME_BIN" ]; then candidates+=("$MOSAIC_RUNTIME_BIN"); fi
+  # A host with no system Node gets one bootstrapped here by tools/install.sh, which
+  # records it in ~/.profile. The fleet unit runs `env -i ... bash --noprofile --norc`
+  # by design, so ~/.profile is never read and the directory has to be named here.
+  # The npm probe below cannot cover this: it reports a package prefix
+  # (~/.npm-global), never a Node runtime directory. It is first so the bootstrapped
+  # runtime wins on a host that has both — that is the one the installer verified.
+  # Runtime binaries are `#!/usr/bin/env node`, so without this the pane resolves the
+  # binary and then dies on `env: 'node': No such file or directory`.
+  candidates+=("$PANE_HOME/.mosaic/node/current/bin")
   if command -v npm >/dev/null 2>&1; then
     local npm_prefix
     npm_prefix=$(npm config get prefix 2>/dev/null) || true