docs: clarify file URL TTL and cleanup

[Abhishek Patil (abhishekpatil4)]

Summary

• document default presigned file URL TTL and 24-hour staged-file cleanup in the Files API overview
• clarify Remote sandbox /mnt/files cleanup and sandbox inactivity lifecycle
• add lightweight cross-links from session and RemoteFile docs to the sandbox retention details

Validation

• bun run generate:api-index
• bun run lint:links
• bun run test
• bun run build

Note: bun run build passed with existing warnings about missing COMPOSIO_API_KEY for detailed toolkit fetches and repeated Fumadocs OpenAPI v3 document warnings.

[vercel]

Abhishek Patil (@abhishekpatil4) is attempting to deploy a commit to the Composio Team on Vercel.

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