feat(web): Prism spawns, collapsed child tree and parent breadcrumb (#17)

[lukemaj]

Closes #17.

The Agents panel listed every child thread as a direct spawn, so threads Model Router (Prism) started for a planner looked like the planner's own spawns. It also had no way to see a child's own children, and a child thread's header gave no route back to its parent.

What changed

• Prism spawns section. Router threads are recognized from the [model-router job <id> <kind> on route <route>; planner thread <id>] tag in their first user message. A thread's shell carries no such metadata, and router titles are regenerated after the first turn, so the tag is the only stable marker. The panel reads each child thread once through the normal thread-detail atom, caches the parsed tag (or "not a router thread") in memory, then drops the subscription. Router threads are grouped by job; the job label shows request id, aggregate status and route (the dispatcher's route). Everything else stays under "Direct spawns". A thread whose tag is not read yet shows under Direct spawns until the read completes.
• Collapsed tree. A row whose thread has live child threads (by parentThreadId if a shell ever carries it, else the sub.<parent>.<suffix> id) shows a ▸ N chevron, collapsed by default. Clicking it reveals compact one-line rows (status dot, title, model), each with its own chevron for deeper levels. Nothing expands on its own. Rows still open their thread.
• Header breadcrumb. A child thread's header reads project / parent title / child title. The parent title links to the parent, and a small menu next to it switches to sibling child threads. Threads the user started are unchanged.

Logic lives in fork-owned files: AgentThreadTree.logic.ts (pure functions plus tests), AgentThreadTree.tsx and chat/ThreadParentCrumbs.tsx. Upstream edits: AgentsPanel.tsx (already allowlisted) and one import plus one JSX line in chat/ChatHeader.tsx (added to scripts/fork-upstream-edits.txt and docs/fork.md).

Not in scope: opening harness-internal subagents read-only (deferred per the Issue), and the mobile client, which has no Agents panel for child threads yet.

Proof

Local, in the worktree (Node 26, pnpm 11.10.0, ELECTRON_RUN_AS_NODE unset):

• pnpm install --frozen-lockfile: pass
• pnpm exec vp run -r typecheck: pass
• pnpm exec vp lint: pass (no warnings in touched lines)
• pnpm exec vp fmt --check: pass
• vp test run AgentThreadTree.logic.test.ts subagentThreads.test.ts chat/ChatHeader.test.ts: 20 tests pass (tag parsing, section assignment, tree nesting and toggle state, shell status, breadcrumb links)
• scripts/fork-check.sh: OK

Live, on an isolated dev instance (server 127.0.0.1:3791, T3CODE_HOME=/tmp/agents-panel-t3home seeded with a VACUUM INTO snapshot of real data, headless Chrome with a fresh profile). Two grandchild threads were created under the router dispatcher thread with a normal thread.create command to exercise nesting.

• Planner thread with job router-80-release-t3: the panel shows Prism spawns with router-80-release-t3 · Failed · luna/max and the dispatcher plus two worker rows; no Direct spawns section, since the planner spawned nothing itself.
• The dispatcher row shows a collapsed ▸ 2 chevron (aria-expanded=false, 0 child rows). Clicking it shows Dispatcher helper: run proof and Dispatcher helper: review diff (both gpt-5.6-luna · max); clicking again hides them (0 rows).
• Dispatcher thread header: model-router / Continue Chromeria Handoff Plan / Fix Release Verification False Failures. The sibling menu lists the two worker threads; choosing one opened sub.…f40b7340, and clicking the parent crumb returned to the planner thread.

Screenshots were kept local because they contain private job prompts and paths from real data: /tmp/agents-panel-shots/01-prism-spawns-collapsed.png, 02-child-tree-expanded.png, 03-child-breadcrumb.png, 04-sibling-menu.png.

Built with Claude Code (Claude Opus 5.5).

🤖 Generated with Claude Code

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	+2 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	0 B (0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+2 B (+0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	56.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	+6 B (+0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−6 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.5 KiB	+12 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 43daf00 · PR result: c9f8dbc · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.