chore: introduce codex-common crate (#843)

I started this PR because I wanted to share the `format_duration()`
utility function in `codex-rs/exec/src/event_processor.rs` with the TUI.
The question was: where to put it?

`core` should have as few dependencies as possible, so moving it there
would introduce a dependency on `chrono`, which seemed undesirable.
`core` already had this `cli` feature to deal with a similar situation
around sharing common utility functions, so I decided to:

* make `core` feature-free
* introduce `common`
* `common` can have as many "special interest" features as it needs,
each of which can declare their own deps
* the first two features of common are `cli` and `elapsed`

In practice, this meant updating a number of `Cargo.toml` files,
replacing this line:

```toml
codex-core = { path = "../core", features = ["cli"] }
```

with these:

```toml
codex-core = { path = "../core" }
codex-common = { path = "../common", features = ["cli"] }
```

Moving `format_duration()` into its own file gave it some "breathing
room" to add a unit test, so I had Codex generate some tests and new
support for durations over 1 minute.

codex-rs/exec/src/event_processor.rs

@@ -1,4 +1,5 @@
 use chrono::Utc;
+use codex_common::elapsed::format_elapsed;
 use codex_core::protocol::Event;
 use codex_core::protocol::EventMsg;
 use codex_core::protocol::FileChange;
@@ -145,7 +146,7 @@ impl EventProcessor {
                 }) = exec_command
                 {
                     (
-                        format_duration(start_time),
+                        format!(" in {}", format_elapsed(start_time)),
                         format!("{}", escape_command(&command).style(self.bold)),
                     )
                 } else {
@@ -160,7 +161,7 @@ impl EventProcessor {
                     .join("\n");
                 match exit_code {
                     0 => {
-                        let title = format!("{call} succeded{duration}:");
+                        let title = format!("{call} succeeded{duration}:");
                         ts_println!("{}", title.style(self.green));
                     }
                     _ => {
@@ -221,7 +222,7 @@ impl EventProcessor {
                     ..
                 }) = info
                 {
-                    (format_duration(start_time), invocation)
+                    (format!(" in {}", format_elapsed(start_time)), invocation)
                 } else {
                     (String::new(), format!("tool('{call_id}')"))
                 };
@@ -335,7 +336,7 @@ impl EventProcessor {
                 }) = patch_begin
                 {
                     (
-                        format_duration(start_time),
+                        format!(" in {}", format_elapsed(start_time)),
                         format!("apply_patch(auto_approved={})", auto_approved),
                     )
                 } else {
@@ -383,13 +384,3 @@ fn format_file_change(change: &FileChange) -> &'static str {
         } => "M",
     }
 }
-
-fn format_duration(start_time: chrono::DateTime<Utc>) -> String {
-    let elapsed = Utc::now().signed_duration_since(start_time);
-    let millis = elapsed.num_milliseconds();
-    if millis < 1000 {
-        format!(" in {}ms", millis)
-    } else {
-        format!(" in {:.2}s", millis as f64 / 1000.0)
-    }
-}