Updates CHANGELOG & README
Refs: #2641

Co-authored-by: Ramin Tadayon <ramin.tadayon@gitkraken.com>

- Moves AI settings into a new AI section
 - Renames new setting into a `gitlens.ai` namespace and changes it to be an enum for easier selection
 - Updates max characters to be dependent on selected model

Closes #2252
Closes #2448
Follow-up to 5f95876a97

Uses the core git fetch command instead of a command-line
git fetch so VsCode will prompt the user for their username
and password. Pull & push also use their corresponding core commands.

In case of multiple cursors (selections) in the same line, an
annotation was created per selection, resulting in an odd experience.
Filter out duplicated lines by the line number.

Add author to CHANGELOG and README

Fixes: #2346

- updates README
- minor updates to GL+ walkthrough intro

- Adds the Commit Graph
- Adds the Commit Details view