Update Agent and skill for What's New generation: filename & issue number instructions

[wadepickett]

The SKILL.md file used by the following agent needs some corrections:
.github/agents/whats-new-from-release-notes.agent.md

The SKILL.md file to correct is here:
.github/skills/whats-new-include-content-rules/SKILL.md

The SKILL.md file needs an update for two issues, do not rewrite the SKILL.md file more than needed to correct the following two issues:

1. Include file names need to be whole words with dashes in between.

This following is an example of a file name that is incorrect since there is word in the filename that is not a whole word, and the number 2 is combined with a word rather than sperated by a dash:
infer-pk-display-name-preview2.md

This is the correct name for the same file:
infer-passkey-display-name-preview-2.md

This section int he SKILL.md file needs to be updated accordingly:
### File naming convention

2. The original issue the PR fixes needs to have a "Fixes #{issue number} followed by a new blank line at the very top of the PR description.

For exmple:
Fixes #10000

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Associated WorkItem - 578457