fix: share single Postgres connection pool across all sessions

[sjawhar]

Problem

Each OpenCode session that triggers Plugin.init() creates its own postgres() connection (with max: 1). On sami-agents-mx with many concurrent sessions, this produces 82+ connections from a single machine. With 4 fleet machines syncing, this will exceed even 300 max_connections.

Evidence

SELECT count(*) FROM pg_stat_activity WHERE datname = 'opencode_test';
-- 82 (from sami-agents-mx alone)

Postgres max_connections was bumped from 100 to 300 as a workaround.

Fix

The plugin's server() function in src/index.ts creates a new postgres(url, { max: 1 }) on every call. Since server() is called per-directory bootstrap, each project directory gets its own connection.

Fix: use a module-level singleton keyed by connection URL. If the URL matches an existing pool, reuse it.

const pools = new Map<string, ReturnType<typeof postgres>>()

function pool(url: string) {
  const hit = pools.get(url)
  if (hit) return hit
  const sql = postgres(url, { max: 3 })
  pools.set(url, sql)
  return sql
}

This drops from N connections (one per directory) to a single shared pool with 3 connections max.

Also fix

• Part FK race: replay failed insert or update on table "part" violates foreign key constraint "part_message_id_fkey" — parts arriving before their parent message. Buffer or use ON CONFLICT DO NOTHING and retry on next event.