Feature Request: allow the host application to supply files and documents as session context

## Problem

A session can be given a prompt and a repository. It cannot be given the surrounding material that motivated the work: a specification, a design document, a log file, a crash dump, a screenshot, or a prior conversation transcript. That material typically lives outside source control and should not be committed to the repository merely to make the agent aware of it.

Today the only reliable workaround is to paste the content into the prompt, which loses fidelity and structure for anything that is not plain text, or to commit the file to the repository, which pollutes history with transient artifacts.

## What is missing

- No API for attaching arbitrary files or binary content to a session as first-class resources.
- No way to supply an external transcript or prior conversation as additional grounding.
- No feedback about which supplied resources were actually read, skipped, or rejected.

## Proposed behavior

- Session creation and subsequent turns accept a list of context resources, each with content or a resolvable reference, a MIME type, and a display name.
- Supplied resources are available to the agent for reading and are distinguishable from repository files.
- The session reports, per resource, whether it was ingested successfully; failures state the reason, such as unsupported type, size limit, or missing permission.
- Size and count limits are documented and enforced predictably.

## Example scenario

A caller supplies a design document and two log files alongside a prompt asking the agent to determine whether the implementation matches the design and to explain a failure visible in the logs.

## Acceptance criteria

- Files and binary content can be attached to a session without committing them to the repository.
- Attached resources are readable by the agent and reported in the session result.
- Ingestion failures are explicit and actionable.

## Related

- #2349 — attachments silently ignored in one command path; this request covers the general capability.

Issue にはファイル、テスト、エントリポイントが特定されていません。まずセッション作成と後続ターンの API を見つけ、次にリソースがどのように取り込まれ、報告されるかを追跡します。添付されたテキストおよびバイナリリソースがリポジトリへのコミットなしで機能し、サポートされていないリソース、サイズが大きすぎるリソース、アクセスできないリソースから対応可能な結果が得られれば完了です。

索引モデルが issue の本文から書いたものです。