Add Opus 4.8 migration guide and model updates to claude-api skill (#1216)

* Add Managed Agents self-hosted sandboxes + mid-session agent updates + MCP tool-output offload to claude-api skill

Self-hosted sandboxes: new shared/managed-agents-self-hosted-sandboxes.md for config:{type:"self_hosted"} — agent loop on Anthropic's orchestration, tool execution on customer infra via outbound-polling worker. Covers EnvironmentWorker.run()/.run_one() (Py/TS), ant beta:worker poll/run, mid-level work.poller()/WorkPoller (Py/TS/Go only; Go has no auto_stop opt-out), AgentToolContext/beta_agent_toolset/tool_runner(), monitoring (environments.work.stats/stop — x-api-key, call from outside worker host), runtime deps, cloud-vs-self_hosted delta table, credentials, security ownership split. Cross-refs in environments.md, overview.md (Reading Guide + rewrote cloud-only pitfall), api-reference.md (SDK row + naming-quirks + schema + work REST rows), tools.md (Who-runs-it carve-out), onboarding.md, live-sources.md.

Mid-session agent updates: sessions.update(session_id, agent={tools, mcp_servers}, vault_ids=[...]) — session-local override (doesn't bump agent version), full-replacement semantics, session must be idle. New core.md section + pointers in tools.md, api-reference.md (UpdateSession row), overview.md.

Large MCP tool outputs → files: >100K tokens → automatic offload to sandbox file; agent gets truncated preview + path. Plus: invalid vault credentials don't block sessions.create() — session.error event fires, auth retries on next idle→running. Both in tools.md.

* Point ant CLI install ref to live-sources.md (OSS has no anthropic-cli.md)

* Add Opus 4.8 model migration guide to claude-api skill

* Add prescriptive tool-description guidance for Opus 4.8 to claude-api skill

skills/claude-api/php/managed-agents/README.md

@@ -48,7 +48,7 @@ use Anthropic\Beta\Agents\BetaManagedAgentsAgentToolset20260401Params;
 // 1. Create the agent (reusable, versioned)
 $agent = $client->beta->agents->create(
     name: 'Coding Assistant',
-    model: 'claude-opus-4-7',
+    model: 'claude-opus-4-8',
     system: 'You are a helpful coding assistant.',
     tools: [
         BetaManagedAgentsAgentToolset20260401Params::with(
@@ -299,7 +299,7 @@ use Anthropic\Beta\Sessions\BetaManagedAgentsAgentParams;
 // Agent declares MCP server (no auth here — auth goes in a vault)
 $agent = $client->beta->agents->create(
     name: 'GitHub Assistant',
-    model: 'claude-opus-4-7',
+    model: 'claude-opus-4-8',
     mcpServers: [
         BetaManagedAgentsUrlmcpServerParams::with(
             type: 'url',