Update claude-api skill: scheduled deployments, vault env-var credentials, system.message events (#1297)

- Add Managed Agents scheduled deployments: new
  shared/managed-agents-scheduled-deployments.md (cron schedules, deployment
  runs, pause/auto-pause), Deployments and Deployment Runs API reference,
  beta-header coverage, and SKILL.md routing
- Vault environment_variable credentials: secrets substituted at egress with
  networking allowlists; secrets guidance in tools, client-patterns, and
  onboarding docs rewritten around them; self-hosted sandbox caveats noted
- Add system.message event for mid-session system prompt updates (Opus 4.8
  only) to the events guide and API reference

skills/claude-api/shared/managed-agents-core.md

@@ -132,7 +132,7 @@ const session = await client.beta.sessions.create(
 | `environment_id`| string   | **Yes**  | Environment ID                                 |
 | `title`         | string   | No       | Human-readable name (appears in logs/dashboards) |
 | `resources`     | array    | No       | Files, GitHub repos, or memory stores, attached to the container at startup. Memory stores are session-create-only (not addable via `resources.add()`). |
-| `vault_ids`     | array    | No       | Vault IDs (`vlt_*`) — MCP credentials with auto-refresh. See `shared/managed-agents-tools.md` → Vaults. |
+| `vault_ids`     | array    | No       | Vault IDs (`vlt_*`) — MCP credentials with auto-refresh + `environment_variable` secrets substituted at egress. See `shared/managed-agents-tools.md` → Vaults. |
 | `metadata`      | object   | No       | User-provided key-value pairs                  |
 
 **Agent configuration fields** (passed to `agents.create()`, not `sessions.create()`):