feat: add path field to ParsedCommand::Read variant

[bolinfest]

ParsedCommand::Read has a name field that attempts to identify the name of the file being read, but the file may not be in the cwd in which the command is invoked as demonstrated by this existing unit test:

codex/codex-rs/core/src/parse_command.rs

Lines 250 to 260 in 0139f67

	#[test]
	fn supports_cat_sed_n() {
	let inner = "cat tui/Cargo.toml | sed -n '1,200p'";
	assert_parsed(
	&vec_str(&["bash", "-lc", inner]),
	vec![ParsedCommand::Read {
	cmd: inner.to_string(),
	name: "Cargo.toml".to_string(),
	}],
	);
	}

As you can see, tui/Cargo.toml is the relative path to the file being read.

This PR introduces a new path: PathBuf field to ParsedCommand::Read that attempts to capture this information. When possible, this is an absolute path, though when relative, it should be resolved against the cwd that will be used to run the command to derive the absolute path.

This should make it easier for clients to provide UI for a "read file" event that corresponds to the command execution.