Feat/interactive provider model selection

[Nithwin]

Summary / What Changed?

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Why? (Motivation & Context)

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Related Issue(s)

• Fixes #

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How Was This Tested?

• Unit Tests (go test -v ./...) — All existing and new unit/golden tests pass.
• Manual TUI / CLI Verification — Ran windmist ... locally to confirm UI behavior and prompt handling.
• Provider API Verification — Tested against live or mock LLM provider endpoints (e.g., Gemini, OpenAI).

Test Environment & Commands Executed:

# Paste exact test commands or output snippets here if applicable

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Checklist

• My code follows the code style and guidelines outlined in CONTRIBUTING.md (or root CONTRIBUTING.md).
• I have verified that the project builds cleanly (go build -v ./...) without errors or warnings.
• I have added or updated unit tests / golden tests for any new functionality or bug fixes.
• I have updated relevant documentation (README.md, docs/, or CLI --help text) if this PR changes user-facing behavior or flags.
• My commit messages are clean, descriptive, and follow conventional commit formats where applicable.
• I have verified that no API keys, tokens, or sensitive credentials are inadvertently included in this branch.