feat: add multi-agent team coordination primitives

[pamelia]

Issue for this PR

Closes #12711
Closes #12661
Closes #15035
Closes #17994

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds multi-agent team coordination to opencode so a lead agent can orchestrate parallel specialist sub-agents that communicate findings and produce synthesized results. This is the infrastructure that powers skills like spec-review and self-review-loop.

Core subsystems added:

• Team management (src/team/) — DB-backed teams with lead/member lifecycle. Members get auto-disambiguated names when duplicate agent types are spawned (e.g., general, general-2). Create and disband are transactional. Members have explicit status transitions: active → completed (normal exit), failed (crash), cancelled (team disbanded).

• Background agent execution (src/tool/task.ts) — Extended TaskTool with background and team_id params. Instance.bind() preserves ALS context. Process-wide concurrency limit (team.max_agents, default 10). When a background agent crashes, the lead gets an injected [AGENT FAILURE] notification and the agent's tasks cascade to failed.

• Cross-session messaging (src/session/inject.ts) — Synthetic user messages written to target session's DB. The prompt loop picks them up via typed Bus.subscribe. Race-safe: subscribes to Bus first, then checks DB for already-pending messages. Target session's agent type is resolved dynamically (not hardcoded).

• Agent memory (src/agent/memory.ts) — Persistent per-agent per-project memory in SQLite. 100KB cap with byte-safe UTF-8 truncation. Injected into system prompt when memory: "local" is set on agent config.

• Team task board (src/team/task.ts) — Shared task state scoped to a team. Tools validate team exists and is active before operating.

• Prompt loop changes (src/session/prompt.ts) — Background team members wait for injected messages (configurable timeout via team.member_timeout). Members mark themselves completed on normal exit. Session cancellation auto-disbands owned teams. Server startup reconciles stale active teams.

• TUI sidebar — New collapsible Teams section shows active teams with member agents and status indicators (working/done/failed/cancelled). Wired to Bus events for real-time updates.

• 5 new tools: team_create, team_delete, team_task, send_message, agent_memory — all permission-gated at execution time.

• Config: New team section with max_agents (int, default 10) and member_timeout (ms, default 300000).

The design follows existing opencode patterns: DB-backed state, Bus pub/sub, Instance.bind for ALS, permission gating. Migration is additive only (4 new tables).

How did you verify your code works?

• Ran bun typecheck from packages/opencode — passes (only pre-existing error in index.tsx unrelated to this PR)
• Tested the full spec-review workflow: created a team, spawned 5 specialist agents in parallel, they reviewed a spec, sent findings via send_message, lead mediated cross-review, produced synthesized output
• Tested the self-review-loop workflow: spawned fresh code-review sub-agents across 3 turns, triaged findings, applied fixes, committed and pushed
• Tested failure handling: verified crashed agents notify the lead and cascade task status
• Tested team lifecycle: session cancellation disbands teams, server restart reconciles stale teams
• Tested TUI: sidebar updates in real-time as teams are created, members join, and statuses change

Screenshots / recordings

N/A — this is backend infrastructure + a small TUI sidebar addition

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found 2 potential duplicate PRs:

1. PR feat: add team tools, HTTP routes, and tool registry integration #12731: feat: add team tools, HTTP routes, and tool registry integration

   • feat: add team tools, HTTP routes, and tool registry integration #12731
   • Related: Adds team tools which overlaps with the 5 new tools in PR feat: add multi-agent team coordination primitives #18753

2. PR feat(opencode): add agent teams — DB-based parallel multi-session collaboration #15205: feat(opencode): add agent teams — DB-based parallel multi-session collaboration

   • feat(opencode): add agent teams — DB-based parallel multi-session collaboration #15205
   • Related: Directly addresses agent teams with DB-backed state and parallel agent execution, which are core features of PR feat: add multi-agent team coordination primitives #18753

These older PRs appear to be addressing similar functionality around team management and multi-agent coordination. Recommend reviewing their status and implementation to check for overlap or conflicts with PR #18753.