fix(sidebar): attach subagent children to sessions rendered inside slug groups

[JeanBaptisteRenard]

Summary

• buildSlugGroup() appended its sessions via the raw buildSessionItem(session), never through appendSubagentChildren — only the ungrouped top-level render path called that helper. Any session rendered inside a slug group (e.g. a schedule rerun sharing a slug with an earlier run — the only real producer of grouped top-level sessions, see schedule-runner.js:createScheduleSession()) silently lost its subagent caret/children.
• Knock-on bug: a slug-group <div> carries no dataset.sessionId of its own, so the "orphan subagents" pass in buildSessionsList never counted a grouped session's id as accounted-for either — its subagents were duplicated into the project's "Orphan subagents" bucket.
• Fix: appendSubagentChildren() is hoisted from a closure inside renderProjects() to module scope (it never captured any of that function's locals) so buildSlugGroup(slug, sessions, subagentIndex) can call it per rendered session, and a new collectTopLevelSessionIds(el) walks into a group element for the orphan-detection pass.
• Design notes and rationale live in .ai/contexts/subagent-observability.md (new section); the code carries only one-line pointers to it.

This is PR 2 of the slug-grouping rehab plan (.work-files/switchboard/slug-grouping-investigation/rehab-plan.md, Option A: A3 nesting fix + A4 first real test of schedule-rerun grouping). PR 1 (A1+A2, dead plan-accept detection cleanup) is separate.

Test plan

• New test/dom-slug-group-subagent-nesting.test.js: seeds two top-level sessions sharing a slug in the exact session_cache row shape schedule-runner.js:createScheduleSession() produces for two reruns of the same schedule, plus a subagent parented to one of them.
  • Confirmed failing on both assertions (missing caret, duplicated orphan bucket) against the unfixed code before applying the fix — pins the bug per rehab-plan.md A4.
  • All 3 assertions pass after the fix: caret/children attached inside the group, no orphan-bucket duplication, group dot lights up while the nested session's PTY is active.
• The two existing Fix1 morphdom fixtures in test/dom-sidebar-perf.test.js that use buildSlugGroup's output as a stateful-DOM fixture still pass unmodified (only the internal signature changed; they exercise it through renderProjects, not directly).
• task check (lint + full suite): 580 passed, 1 pre-existing skip, 0 failures.

Refs #128 (Part 2 of the cleanup plan)