fix(worker): replace script -qfc with pty-process for injection-safe PTY (#1678)

- Add pty-process crate (MIT, tokio async support) for PTY allocation
- Spawn claude CLI with pty-process::Command::arg() chaining instead of
  building a shell string for script -qfc
- Eliminates all shell injection surfaces: prompt, model, session_id
  are passed via execve, never interpreted by a shell
- Keep stderr on separate pipe to prevent NDJSON parse breakage
  (pty-process attaches PTY to all fds by default)
- Gate PTY behind #[cfg(unix)] with direct-spawn fallback for Windows CI
- Read stdout from PTY master (implements tokio::io::AsyncRead)
- Add regression tests: arg vector construction + PTY allocation

Addresses review feedback from zmanian and gemini-code-assist.

Co-authored-by: j-bloggs <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>

Cargo.lock

@@ -3150,7 +3150,7 @@ dependencies = [
  "libc",
  "percent-encoding",
  "pin-project-lite",
- "socket2 0.5.10",
+ "socket2 0.6.3",
  "system-configuration",
  "tokio",
  "tower-service",
@@ -3439,6 +3439,7 @@ dependencies = [
  "pgvector",
  "postgres-types",
  "pretty_assertions",
+ "pty-process",
  "rand 0.8.5",
  "readabilityrs",
  "refinery",
@@ -3524,7 +3525,7 @@ checksum = "3640c1c38b8e4e43584d8df18be5fc6b0aa314ce6ebf51b53313d4306cca8e46"
 dependencies = [
  "hermit-abi",
  "libc",
- "windows-sys 0.59.0",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]
@@ -4906,6 +4907,16 @@ dependencies = [
  "syn 1.0.109",
 ]
 
+[[package]]
+name = "pty-process"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "71cec9e2670207c5ebb9e477763c74436af3b9091dd550b9fb3c1bec7f3ea266"
+dependencies = [
+ "rustix 1.1.4",
+ "tokio",
+]
+
 [[package]]
 name = "pulley-interpreter"
 version = "28.0.1"
@@ -4930,7 +4941,7 @@ dependencies = [
  "quinn-udp",
  "rustc-hash 2.1.1",
  "rustls 0.23.37",
- "socket2 0.5.10",
+ "socket2 0.6.3",
  "thiserror 2.0.18",
  "tokio",
  "tracing",
@@ -4967,9 +4978,9 @@ dependencies = [
  "cfg_aliases",
  "libc",
  "once_cell",
- "socket2 0.5.10",
+ "socket2 0.6.3",
  "tracing",
- "windows-sys 0.59.0",
+ "windows-sys 0.60.2",
 ]
 
 [[package]]