ci: Restore severity prefix on claude-review inline comments

ci: Restore severity prefix on claude-review inline comments

Commit a65ebc3ff9 ("claude-review: improve review quality for large
PRs") dropped the `Claude: **<severity>**: ` prefix from posted inline
comments on the theory that Claude was also adding the severity into
`body`, producing duplicates. But nothing in the prompt or schema
actually asks the subagent to include severity in `body` — severity
is a separate structured field. The result is that inline comments
no longer show must-fix/suggestion/nit classification.

Restore the prefix in the posting step, and add an explicit instruction
to the subagent prompt telling it not to repeat severity inside `body`
so the two don't collide.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>