feat: sub-agent tree — collapsible parent nodes with ▶/▼ toggle (closes #33)

[vivekchand]

What\n\nAdds a dedicated Sub-Agent Tree page to the ClawMetry dashboard with collapsible parent nodes.\n\n## Changes\n\n- New route — filters sessions to sub-agent entries, adds depth/parent fields, computes status (active/idle/stale) and runtime string\n- Nav tab — "Agents" tab added to both dashboard themes\n- page-subagents div — follows the pattern of page-transcripts/page-clusters in both themes\n- loadSubagents() JS function — fetches /api/subagents, builds parent-child tree using parent/depth fields, renders with ▶/▼ toggle buttons per node\n- Auto-refresh — setInterval at 5s while Agents tab is active; clears on tab switch\n\n## UX\n\n- ▼/▶ toggle collapses/expands subtrees (state persisted in _subagentsExpanded map)\n- Status dots: 🟢 active (<2min), 🟡 idle (<10min), ⬜ stale\n- Shows: agent name, model, token count, depth badge, runtime\n- Summary bar: total / active / idle / stale counts\n\nCloses #33