Add pre-release build workflows

[4pmtong]

Related Issue

Closes #

Description

• Add manual pre-release build workflow for validation builds
• Add tag-based pre-release workflow for generating release artifacts
• Store pre-release artifacts under separate S3 prefixes

Notes

• Existing build workflows are unchanged
• Pre-release outputs are isolated from normal release paths

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