feat: add configurable remote sub agents

[4pmtong]

Related Issue

Closes #

Description

Adds configurable Remote Sub Agents support.

Changes include:

• Adds a Sub Agents settings page alongside Models, Skills, and Memory.
• Adds Gemini Agents as the first Remote Sub Agent provider.
• Lets users configure provider credentials, endpoint, agent ID, and runtime limits from the UI instead of .env.
• Adds provider validation before saving/enabling the config.
• Enables RemoteSubAgent toolkit only when the user has configured and enabled a valid provider.
• Adds runtime prompt guidance so eligible long-running or isolated tasks can be delegated to RemoteSubAgent.
• Updates RemoteSubAgent provider defaults, localization, and tests.

Testing Evidence (REQUIRED)

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline