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/tui/src/cli.rs

@@ -1,6 +1,6 @@
 use clap::Parser;
-use codex_core::ApprovalModeCliArg;
-use codex_core::SandboxPermissionOption;
+use codex_common::ApprovalModeCliArg;
+use codex_common::SandboxPermissionOption;
 use std::path::PathBuf;
 
 #[derive(Parser, Debug)]

codex-rs/tui/src/history_cell.rs

@@ -1,4 +1,5 @@
 use codex_ansi_escape::ansi_escape_line;
+use codex_common::elapsed::format_duration;
 use codex_core::config::Config;
 use codex_core::protocol::FileChange;
 use ratatui::prelude::*;
@@ -132,7 +133,12 @@ impl HistoryCell {
         // Title depends on whether we have output yet.
         let title_line = Line::from(vec![
             "command".magenta(),
-            format!(" (code: {}, duration: {:?})", exit_code, duration).dim(),
+            format!(
+                " (code: {}, duration: {})",
+                exit_code,
+                format_duration(duration)
+            )
+            .dim(),
         ]);
         lines.push(title_line);
 
@@ -201,11 +207,11 @@ impl HistoryCell {
         success: bool,
         result: Option<serde_json::Value>,
     ) -> Self {
-        let duration = start.elapsed();
+        let duration = format_duration(start.elapsed());
         let status_str = if success { "success" } else { "failed" };
         let title_line = Line::from(vec![
             "tool".magenta(),
-            format!(" {fq_tool_name} ({status_str}, duration: {:?})", duration).dim(),
+            format!(" {fq_tool_name} ({status_str}, duration: {})", duration).dim(),
         ]);
 
         let mut lines: Vec<Line<'static>> = Vec::new();