fix(fleet): bake MOSAIC_AGENT_NAME into the agent pane so native HB fires

Live-validation (Lead, w-jarvis) found the native heartbeat was INERT in
production: the Pi extension gates on MOSAIC_AGENT_NAME, but tmux panes
inherit the tmux SERVER environment (not this script's env, nor the systemd
unit's), so the name was empty in-pane for BOTH ad-hoc and systemd agents.
Result: no native .hb, no model self-report — only the sidecar fallback ran.

Fix: %q-quote the agent name and export it into the pane command alongside
PATH, so the extension sees it -> nativeHbEnabled() -> writes <name>.hb with
model + busy/ok turn state.

Re-validated live via the launcher (isolated socket, real pi on glm-5.2):
  - pane env now carries MOSAIC_AGENT_NAME
  - <name>.hb written with status=ok + model=glm-5.2 + .hb.native marker
  - status flips ok -> busy on a real turn -> ok on turn end
  - sidecar defers to the fresh native marker

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EsgTQzV5YUGk1JtCLP4B83

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

@@ -90,11 +90,18 @@ MOSAIC_RUNTIME_BIN_PREFIX=$(_build_runtime_bin_prefix)
 #
 # We build the snippet as a double-quoted here-string embedded in a printf call
 # to avoid nested quoting problems.
+#
+# MOSAIC_AGENT_NAME must also be exported INTO the pane: panes inherit the tmux
+# server environment (not this script's, and not the systemd unit's), so the
+# name would otherwise be empty in-pane and the runtime's native heartbeat
+# (which gates on MOSAIC_AGENT_NAME) would never fire. %q-quote it so it is a
+# safe single bash token regardless of the name's characters.
+AGENT_NAME_Q=$(printf '%q' "$AGENT_NAME")
 
 if [ -n "$MOSAIC_RUNTIME_BIN_PREFIX" ]; then
-  PANE_SHELL_SNIPPET="export PATH=\"${MOSAIC_RUNTIME_BIN_PREFIX}:\${PATH}\"; exec ${MOSAIC_AGENT_COMMAND}"
+  PANE_SHELL_SNIPPET="export MOSAIC_AGENT_NAME=${AGENT_NAME_Q}; export PATH=\"${MOSAIC_RUNTIME_BIN_PREFIX}:\${PATH}\"; exec ${MOSAIC_AGENT_COMMAND}"
 else
-  PANE_SHELL_SNIPPET="exec ${MOSAIC_AGENT_COMMAND}"
+  PANE_SHELL_SNIPPET="export MOSAIC_AGENT_NAME=${AGENT_NAME_Q}; exec ${MOSAIC_AGENT_COMMAND}"
 fi
 
 mkdir -p "$MOSAIC_AGENT_WORKDIR"