First-party Agent Skills support in langchain (without depending on deepagents)

### Submission checklist - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Package (Required) - [x] langchain ### Feature Description Follow-up to #34548 ("Support Agent Skills"). The resolution there was that Agent Skills (the [agentskills.io](https://agentskills.io/) open standard) are supported via `deepagents.middleware.SkillsMiddleware`, which is built on LangChain's `AgentMiddleware` and works with `create_agent`. This request is for **first-party Skills support exported from `langchain` itself**, so users do not have to take a dependency on the `deepagents` package to use Skills in a standard LangChain agent / chat application. ### Use Case Many users want to use the Agent Skills standard in a plain LangChain `create_agent` chat application but cannot or do not want to add `deepagents` as a dependency (extra dependency surface, packaging/governance constraints, "Skills is a core agent capability, not a Deep Agents add-on"). Today the only supported path is `deepagents.middleware.SkillsMiddleware`. First-party support would let users adopt Skills with `langchain` alone. ### Proposed Solution Promote a Skills middleware into `langchain` (mirroring the existing `deepagents` implementation), e.g.: ```python from langchain.agents import create_agent from langchain.agents.middleware import SkillsMiddleware agent = create_agent( model="openai:gpt-5", tools=[...], middleware=[ SkillsMiddleware(sources=["/path/to/project/skills"]), ], ) ``` Behavior (consistent with the standard / existing deepagents middleware): 1. Discover available skills from the configured source directories. 2. Append skill me…