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/error-codes.md

@@ -80,7 +80,7 @@ This file documents HTTP error codes returned by the Claude API, their common ca
 - Using deprecated model ID
 - Invalid API endpoint
 
-**Fix:** Use exact model IDs from the models documentation. You can use aliases (e.g., `claude-opus-4-7`).
+**Fix:** Use exact model IDs from the models documentation. You can use aliases (e.g., `claude-opus-4-8`).
 
 ---
 
@@ -105,7 +105,7 @@ Some 400 errors are specifically related to parameter validation:
 - `budget_tokens` >= `max_tokens` in extended thinking
 - Invalid tool definition schema
 
-**Model-specific 400s on Opus 4.7:**
+**Model-specific 400s on Opus 4.8 / 4.7:**
 
 - `temperature`, `top_p`, `top_k` are removed — sending any of them returns 400. Delete the parameter; see `shared/model-migration.md` → Per-SDK Syntax Reference.
 - `thinking: {type: "enabled", budget_tokens: N}` is removed — sending it returns 400. Use `thinking: {type: "adaptive"}` instead.
@@ -166,10 +166,10 @@ thinking: budget_tokens=10000, max_tokens=16000
 
 | Mistake                         | Error            | Fix                                                     |
 | ------------------------------- | ---------------- | ------------------------------------------------------- |
-| `temperature`/`top_p`/`top_k` on Opus 4.7 | 400    | Remove the parameter (see `shared/model-migration.md`)  |
-| `budget_tokens` on Opus 4.7     | 400              | Use `thinking: {type: "adaptive"}`                      |
+| `temperature`/`top_p`/`top_k` on Opus 4.8 / 4.7 | 400 | Remove the parameter (see `shared/model-migration.md`)  |
+| `budget_tokens` on Opus 4.8 / 4.7 | 400            | Use `thinking: {type: "adaptive"}`                      |
 | `budget_tokens` >= `max_tokens` (older models) | 400 | Ensure `budget_tokens` < `max_tokens`                  |
-| Typo in model ID                | 404              | Use valid model ID like `claude-opus-4-7`               |
+| Typo in model ID                | 404              | Use valid model ID like `claude-opus-4-8`               |
 | First message is `assistant`    | 400              | First message must be `user`                            |
 | Consecutive same-role messages  | 400              | Alternate `user` and `assistant`                        |
 | API key in code                 | 401 (leaked key) | Use environment variable                                |