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/shared/tool-use-concepts.md

@@ -35,7 +35,7 @@ Each tool requires a name, description, and JSON Schema for its inputs:
 **Best practices for tool definitions:**
 
 - Use clear, descriptive names (e.g., `get_weather`, `search_database`, `send_email`)
-- Write detailed descriptions — Claude uses these to decide when to use the tool
+- Write detailed descriptions — Claude uses these to decide when to use the tool. Be **prescriptive about *when* to call it**, not just what it does (e.g. "Call this when the user asks about current prices or recent events"). On recent Opus models, which reach for tools more conservatively, trigger conditions in the description give measurable lift in should-call rate.
 - Include descriptions for each property
 - Use `enum` for parameters with a fixed set of values
 - Mark truly required parameters in `required`; make others optional with defaults
@@ -74,7 +74,7 @@ if response.stop_reason == "pause_turn":
     ]
     # Make another API request — server resumes automatically
     response = client.messages.create(
-        model="claude-opus-4-7", messages=messages, tools=tools
+        model="claude-opus-4-8", messages=messages, tools=tools
     )
 ```
 
@@ -171,7 +171,7 @@ Web search and web fetch let Claude search the web and retrieve page content. Th
 ]
 ```
 
-### Dynamic Filtering (Opus 4.7 / Opus 4.6 / Sonnet 4.6)
+### Dynamic Filtering (Opus 4.8 / Opus 4.7 / Opus 4.6 / Sonnet 4.6)
 
 The `web_search_20260209` and `web_fetch_20260209` versions support **dynamic filtering** — Claude writes and executes code to filter search results before they reach the context window, improving accuracy and token efficiency. Dynamic filtering is built into these tool versions and activates automatically; you do not need to separately declare the `code_execution` tool or pass any beta header.
 
@@ -280,7 +280,7 @@ Two features are available:
 - **JSON outputs** (`output_config.format`): Control Claude's response format
 - **Strict tool use** (`strict: true`): Guarantee valid tool parameter schemas
 
-**Supported models:** Claude Opus 4.7, Claude Sonnet 4.6, and Claude Haiku 4.5. Legacy models (Claude Opus 4.5, Claude Opus 4.1) also support structured outputs.
+**Supported models:** Claude Opus 4.8, Claude Sonnet 4.6, and Claude Haiku 4.5. Legacy models (Claude Opus 4.5, Claude Opus 4.1) also support structured outputs.
 
 > **Recommended:** Use `client.messages.parse()` which automatically validates responses against your schema. When using `messages.create()` directly, use `output_config: {format: {...}}`. The `output_format` convenience parameter is also accepted by some SDK methods (e.g., `.parse()`), but `output_config.format` is the canonical API-level parameter.