feat(api): add public run attachment upload

[jojotaro1994]

Summary

Adds a thin Public API wrapper around StaffDeck’s existing chat attachment staging and TaskFrame materialization flow.

• Adds POST /agents/{agent_id}/attachments for multipart/form-data with repeated files[] fields.
• Reuses the existing runs:create scope and returns ChatAttachmentRead[].
• Keeps AgentRunCreate.attachments; new and streaming Runs share staged-attachment validation.
• Verifies tenant, actor user, metadata, SHA-256, sandbox path, file count, and configured size before creating a job.
• Reuses materialize_task_attachments() so each TaskFrame receives an isolated workspace copy.
• Documents that follow-up Runs must resend the saved attachment descriptors; attachments are not automatically inherited by Session.

Configuration

CHAT_ATTACHMENT_MAX_BYTES controls the UI upload, Public API upload, and Run validation limit. The default is 12 MiB (12582912 bytes). This remains a finite limit because the current upload path reads the request into memory. Harness file tools and individual Skills may impose lower limits.

Scope

This PR intentionally does not add a database table or migration, new Attachment scope, remote URL fetch, query/delete endpoints, TTL cleanup, Session inheritance, or Plane/Bella-specific logic. Staging remains isolated by tenant, actor user, and attachment ID; formal staging lifecycle management is a known limitation.

Validation

• Focused attachment, Public API, staging, chat attachment, and Harness tests: 131 passed.
• Full backend suite: 2006 passed, 10 failed; the failures are existing environment/service issues: five General Skill runtime dependency tests fail because the generated macOS runtime is missing libpython3.11.dylib, and five tool-auth-matrix service tests fail against the local SQLite/service-auth fixture (human_handoff_requests table errors). No attachment-focused test failed.
• git diff --check: passed.
• Ruff on changed attachment implementation/tests (E,F,I): passed.
• Full ruff check backend remains blocked by the upstream baseline (1987 existing findings).