fix(fleet): complete heartbeat reader/writer consistency + sidecar hardening

F3 follow-on to #595 (HB consistency) — the items flagged in the #595 review:
- defaultMosaicHome() honors MOSAIC_HOME env (not just --mosaic-home flag), so the
  reader matches the writer/launcher when MOSAIC_HOME is set in the shell. The
  systemd guard now checks the LITERAL ~/.config/mosaic (units use %h paths).
- heartbeatPath() honors MOSAIC_HEARTBEAT_RUN_DIR (the writer sidecar's override).
- sidecar: printf %q the interpolated hb path / pid / interval (defense-in-depth).
- vitest: heartbeatPath env-resolution coverage.

Deferred to next F3 milestone (need deeper code work): agent-watch viewer-leak
try/finally fix, and the test-start-agent-session.sh workdir-assumption fix.

Refs #588 #542

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -129,7 +129,7 @@ _start_heartbeat_sidecar() {
   # references to any variables from this script's environment.
   local sidecar_script
   sidecar_script=$(printf \
-    'hb=%s; pid=%s; iv=%s; mkdir -p "$(dirname "$hb")"; while kill -0 "$pid" 2>/dev/null; do tmp="$hb.tmp.$$"; printf "ts=%%s\npid=%%s\nstatus=ok\n" "$(date +%%Y-%%m-%%dT%%H:%%M:%%S%%z)" "$pid" > "$tmp" && mv "$tmp" "$hb"; sleep "$iv"; done' \
+    'hb=%q; pid=%q; iv=%q; mkdir -p "$(dirname "$hb")"; while kill -0 "$pid" 2>/dev/null; do tmp="$hb.tmp.$$"; printf "ts=%%s\npid=%%s\nstatus=ok\n" "$(date +%%Y-%%m-%%dT%%H:%%M:%%S%%z)" "$pid" > "$tmp" && mv "$tmp" "$hb"; sleep "$iv"; done' \
     "$hb_file" "$pane_pid" "$interval")
 
   # setsid + disown ensures the sidecar survives this script exiting.