chore: introduce codex-common crate

[bolinfest]

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:

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

with these:

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.