Rewrite the warnings in webapp/README

[WULCAN]

Description

The warnings in webapp/README.md about trusting target repositories are not clear enough.

lyra/webapp/README.md

Lines 12 to 17 in b92fe73

	> [!WARNING]
	> A party who controls a source code repository branch
	> which Lyra is set up to translate will also have
	> a lot of control over the Lyra process. That control
	> probably includes reading files from the operating system
	> and possibly includes arbitrary code exection.

lyra/webapp/README.md

Lines 22 to 26 in b92fe73

	> [!WARNING]
	> A party who controls the contents of
	> a local repository directory will also have
	> a lot of control over the Lyra process, probably
	> including arbitrary code execution.

These are some of the first things a developer will read about Lyra so its important they are not confusing.

Relevant Job Stories

Estimated size (S, M, L)

I think this issue is small because the problem is very local to the README.

Prerequisites

Understand the risks.

Open questions

The warnings could perhaps be moved to somewhere else, where they don't need to be as well written, but they still need to be improved.