fix(acp): bound newline-delimited input frames

[fontanierh]

Closes #340. Closes #342.

AcpAgent child stdout ─┐
Stdio stdin            ├─▶ bounded reader (16 MiB default / override) ─▶ JSON-RPC
ByteStreams input      ┘
                                  │ oversize
                                  └─▶ InvalidData / transport cleanup

One shared reader bounds memory before UTF-8 and JSON parsing while preserving exact-limit, CRLF and EOF behavior.

Validation: just test; core all-feature Clippy with -D warnings.