docs: clarified worker persistence and functionality of vibes mode

[Ekansh38]

I was implementing 99 into my workflow and got stuck trying to understand some details of how the vibes and work items worked. That prompted me to go read through the codebase to try to understand how they worked. I thought I would update the docs so future users have a easier time.

vibe had "No description" so I added a brief explanation on how it worked.
I was confused how worker worked so I added some info clarifying how the info was stored.

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Note

Low Risk
Low risk documentation-only change clarifying how vibe works and where Worker work items are persisted; no runtime behavior is modified.

Overview
Clarifies the README by documenting vibe as an agentic AI mode that applies changes via the selected provider and reports modified locations via the quickfix list.

Adds a note that _99.Extensions.Worker work items persist across Neovim sessions and are stored under <tmp_dir>/work-item per project directory.

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