fleet(test): attribute FIRST-position to the fixture, not to the fix

Comment-only. The colon-padding note said the anchored regex 'rejects the
directory in FIRST position — which is exactly where the fix puts it'. True in
this fixture, which runs env -i with no MOSAIC_RUNTIME_BIN so the bootstrap
directory leads, but stated as a property of the fix. In general the directory
sits second, after MOSAIC_RUNTIME_BIN, and it reads as a contradiction of the
source comment corrected in c7ee3cb.

Found in review by rhodey, who also noted the same sentence is live in the PR
description. No executable line changed.

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

@@ -422,8 +422,12 @@ ln -sf "$FAKE_BIN/npm" "$NODE_LAUNCHER_BIN/npm"
 node_pane_environment=$(tr '\0' '\n' < "$HOME_NODE/fleet/pane-environment")
 # Colon-pad and match a whole element. A regex with `(^|:)` after `.*` looks like it
 # does this and does not: an anchor cannot match mid-pattern, so it silently requires
-# a leading colon and rejects the directory in FIRST position — which is exactly where
-# the fix puts it. That produced a failure reading "pane ran but PANE_PATH does not
+# a leading colon and rejects the directory in FIRST position — which is where THIS
+# FIXTURE puts it: it runs under `env -i` with no MOSAIC_RUNTIME_BIN, so the bootstrap
+# directory leads. That is a property of the fixture, not of the fix — in general the
+# directory sits second, after MOSAIC_RUNTIME_BIN. The colon padding makes the
+# assertion position-independent either way, which is why it is written this way and
+# not with an anchor. That produced a failure reading "pane ran but PANE_PATH does not
 # carry <dir>" against a PATH whose first element was that dir.
 node_pane_path=":$(printf '%s\n' "$node_pane_environment" | sed -n 's/^PATH=//p' | head -1):"
 case "$node_pane_path" in