feat: derive sub-agent state and surface delegated work

[leoncheng57]

Closes #46. Implements the state model and orchestration surface in #51.

Why

OpenCode has child sessions but no durable background-job API, so "what are this session's sub-agents doing?" cannot be read — it has to be derived. Until now the runner had the primitives (parentID, subagent_depth) and used none of them: children were invisible as children, a delegation was an opaque task chip, and a background hand-back rendered as a chat bubble the human never typed. In one audited project 124 of 149 sessions were children, so the flat list was ~83% noise.

The state model

server/opencode/subagents.ts reconciles four upstream facts into one ledger keyed by child session id — never by task-part id, since resume emits several parts per child:

Evidence	Strength
/session/status reports busy	Observed, process-local
The child's own final turn	First-hand terminal
A hand-back notice in the parent	Second-hand but explicit
The delegating task part	Weakest, and conditional

Precedence follows that order. A background task part flips to completed when the launch call returns, typically long before the child finishes, so it is never terminal evidence. A synchronous one blocked on the child and is.

When nothing settles a child — cancelled, or the agent server restarted mid-run — the row is unknown and says what was checked. The audit found three cancelled children with no parent notification at all; rendering those as "completed" would be a confident falsehood, so the UI admits it instead. Every row shows its provenance.

Surfaces

• Hub lists roots depth-first with their delegated work, a sub pill, and a direct child count. Nested delegation and children whose parent is archived or elsewhere are both preserved — the flatten emits every session exactly once, including through a corrupt parent cycle.
• Child transcript badges itself and links back to the parent that delegated it.
• Task rows link to the child that ran them, and are excluded from collapsed action groups so that route is never hidden behind a chevron.
• Hand-back notices render as status rows instead of prompts attributed to the user. Detection requires a child session id and an outcome word.
• Subagents inspector tab lists every child with state, evidence, agent, cost and a link.
• Deep links can open ?panel=subagents; on mobile this opens the full-screen inspector sheet.

Controls promise only what the connected process can deliver

Stop appears solely for children /session/status reports busy, because abort authority is process-local. Background promotion is gated on /experimental/capabilities, not on an env var the BFF cannot read. Child endpoints verify the parent link first.

Configuration and settings

• Sets this repository's subagent_depth to 3. OpenCode defaults to 1, which prevents nested delegation.
• Displays the global depth read-only on Settings; project-level authoring remains in opencode.json.
• Represents inherited compaction booleans explicitly as Default (on/off) instead of incorrectly rendering an unset value as off.
• Keeps inherited values out of PATCH bodies so saving does not silently pin defaults.

Cost

One child-list and one status call, the newest parent page for intent, and child transcripts probed only for children that are neither running nor already settled — newest first, capped at 12, concurrency-limited, with truncated reported rather than silently implied.

Notes for review

• SessionSummary.childCount is grouped from the directory listing the Hub already fetches, so it costs no extra upstream call.
• No new runtime dependencies.
• Out of scope: Support data-driven agent selection beyond Plan and Build #52 (data-driven agent selection). A child running a non-Plan/Build agent still correctly refuses prompts via the existing foreign-agent guard from fix: protect foreign session permissions #34.

Verification

• npm run typecheck
• npm test (356 passed)
• npm run build
• npm run test:e2e (192 passed, 1 expected screenshot-runner skip)
• npm run screenshots:local (8 routes; desktop + mobile)

full:/?directory=/tmp/mock-subagent-project
full:/sessions/ses_mock_parent?directory=/tmp/mock-subagent-project
full:/sessions/ses_mock_child_running?directory=/tmp/mock-subagent-project
full:/sessions/ses_mock_parent?directory=/tmp/mock-subagent-project&panel=subagents
full:/settings