setup writes unescaped strings into agent TOML config, producing invalid files on Windows

[Rene-Kuhm]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

🐛 Bug Description

When setup writes the agent configuration, string values are inserted into
the generated TOML without escaping. TOML v1.0 basic strings require \ and
" to be escaped, so any value containing them produces a file no compliant
parser will read.

This is not a corner case on Windows: the install paths resolve under
%APPDATA%, so the emitted value contains backslashes by construction.

🔁 Steps to Reproduce

Run the setup on a path containing a backslash or a double quote — on Windows,
the default install path is enough — then parse the generated TOML.

✅ Expected Behaviour

The generated file is valid TOML, and the value read back is byte-for-byte
what was written.

❌ Actual Behaviour

The emitted string is written raw. A path such as
C:\Users\name\AppData\Roaming becomes an invalid escape sequence, and the
config fails to parse.

📦 Affected Area

internal/setup — config writing for the Gemini and Codex integrations.

📎 Additional Context

I have a fix ready: a tomlStringEscape helper applied when writing string
values, with round-trip tests that use a hand-rolled decoder to assert the
emitted value parses back to the original.

Happy to open the PR once this is approved.