feat(delegate): run cursor_delegate as a navigable child session

[justin-carper]

Problem

opencode lets you navigate into and follow subagent calls from a parent session. cursor_delegate did not participate in that UI: it ran the Cursor SDK directly inside the tool and collapsed the whole turn into a single result block, so Cursor's tool calls, reasoning, and text were invisible.

Why this approach

opencode's SDK has no API to inject externally-produced message parts into a session (/global/event is subscribe-only; there is no part-write endpoint). The only way to surface a live, navigable subagent is to let opencode itself drive the turn on a child session via session.create({ parentID }) + session.prompt(...).

What changed

cursor_delegate now (keeping its API-key check and context.ask permission gate up front):

1. Creates a parent-linked child session (parentID = context.sessionID).
2. Runs the delegated prompt through session.prompt against the cursor provider.
3. opencode's existing provider pipeline (language-model.ts → stream-map.ts) streams the child turn, so Cursor's live activity renders in the navigable subagent UI; the child session persists for later inspection.
4. Returns the final text plus metadata { childSessionID, model, status, usage }.

Supporting changes:

• Per-turn control bridge: mode/thinking/sandbox/agentId/cwd are forwarded to only the matching child session via a session-keyed map consumed by the existing chat.params hook, then cleared after the turn (siblings never inherit).
• Provider: reads per-turn sandbox and cwd from providerOptions.cursor; cwd now reaches acquireAgent (not just the session query).
• Abort: aborts the in-flight child on tool abort, and deletes a child that finishes creation after abort won the race so no empty orphan session lingers.
• Removed the now-dead runDelegate runtime (src/provider/delegate.ts) and its tests.

Testing

• npm test — 299 passing (added child-session creation/prompt shape, control forwarding + sibling isolation, per-turn cwd/sandbox, abort + orphan-delete, restored cloud-agent regressions).
• npm run typecheck — clean.
• npm run build — success.

Reviews

Two code-reviewer passes. First surfaced a cwd-propagation gap and an abort race (both fixed); second returned ready-to-merge with one Important item — abort-race child should be deleted, not aborted (a no-op on an unprompted session) — now fixed and covered by a test.

Notes

• Same sandbox posture as before (provider path); the delegated Cursor execution is unchanged, only opencode now owns the turn.
• Host dependency: the child is always reachable via parentID/session.children; whether the parent's tool block is click-through depends on the opencode TUI version. childSessionID is exposed in metadata to support click-through where available. No private TUI APIs used.