Move environment abstraction into exec server (#15125)

The idea is that codex-exec exposes an Environment struct with services
on it. Each of those is a trait.

Depending on construction parameters passed to Environment they are
either backed by local or remote server but core doesn't see these
differences.

codex-rs/exec-server/Cargo.toml

@@ -15,13 +15,16 @@ path = "src/bin/codex-exec-server.rs"
 workspace = true
 
 [dependencies]
+async-trait = { workspace = true }
 clap = { workspace = true, features = ["derive"] }
 codex-app-server-protocol = { workspace = true }
+codex-utils-absolute-path = { workspace = true }
 futures = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true, features = [
+    "fs",
     "io-std",
     "io-util",
     "macros",
@@ -38,3 +41,4 @@ tracing = { workspace = true }
 anyhow = { workspace = true }
 codex-utils-cargo-bin = { workspace = true }
 pretty_assertions = { workspace = true }
+tempfile = { workspace = true }