feat(stats): hive-wide "favorite tools" rollup on ST4TS

Swarm-level companion to the per-agent favorite-tools doughnut (#1433).
Aggregates each agent's bash_commands(ts, head) table (written by
hive-bash-mcp) across the whole hive and surfaces the top-10 most-run
command heads on the dashboard ST4TS tab, alongside the existing model
mix.

- hive_stats.rs: AgentAgg gains a `bash` head→count map, filled by a new
  guarded `read_bash_heads()` that reuses read_agent's read-only
  connection. A missing `bash_commands` table (capture hasn't run for
  that agent) or any read error yields an empty map — isolated from
  read_agent's error path so it never drops an agent from the rollup.
  HiveStats gains `bash_mix: Vec<KeyCount>` (busiest-first, top 10).
  Unit tests cover the per-head tally + window cutoff and the
  missing-table degrade-to-empty path (in-memory sqlite).
- dashboard: a "favorite tools (bash commands across the swarm)" CSS-bar
  list on the ST4TS pane, mirroring the model-mix bars. Header + list
  stay hidden until bash_mix has data, so a fresh hive shows no empty
  block. (Dashboard ships no chart lib — bars, not a doughnut.)
- docs: dashboard.md ST4TS section documents the new rollup.

Closes #1449. Inert until the hive-bash-mcp capture (#1448, merged) has
recorded data across agents.

frontend/packages/dashboard/src/dashboard.html

@@ -289,6 +289,11 @@
       <div id="hive-stats-agents"><p class="meta">loading…</p></div>
       <h3>◇ model mix (turns across the swarm)</h3>
       <div id="hive-stats-models"></div>
+      <!-- "favorite tools": most-run bash commands across the swarm.
+           Header + list hidden until the hive-bash-mcp capture has
+           recorded data, so the section never shows an empty block. -->
+      <h3 id="hive-stats-bash-h" hidden>◇ favorite tools (bash commands across the swarm)</h3>
+      <div id="hive-stats-bash" hidden></div>
     </section>
 
     <!-- FL0W: lives on its own page now (`/flow.html`). The