enable-resume (#3537)

Adding the ability to resume conversations.
we have one verb `resume`. 

Behavior:

`tui`:
`codex resume`: opens session picker
`codex resume --last`: continue last message
`codex resume <session id>`: continue conversation with `session id`

`exec`:
`codex resume --last`: continue last conversation
`codex resume <session id>`: continue conversation with `session id`

Implementation:
- I added a function to find the path in `~/.codex/sessions/` with a
`UUID`. This is helpful in resuming with session id.
- Added the above mentioned flags
- Added lots of testing

codex-rs/tui/src/cli.rs

@@ -13,35 +13,18 @@ pub struct Cli {
     #[arg(long = "image", short = 'i', value_name = "FILE", value_delimiter = ',', num_args = 1..)]
     pub images: Vec<PathBuf>,
 
-    /// Open an interactive picker to resume a previous session recorded on disk
-    /// instead of starting a new one.
-    ///
-    /// Notes:
-    /// - Mutually exclusive with `--continue`.
-    /// - The picker displays recent sessions and a preview of the first real user
-    ///   message to help you select the right one.
-    #[arg(
-        long = "resume",
-        default_value_t = false,
-        conflicts_with = "continue",
-        hide = true
-    )]
-    pub resume: bool,
+    // Internal controls set by the top-level `codex resume` subcommand.
+    // These are not exposed as user flags on the base `codex` command.
+    #[clap(skip)]
+    pub resume_picker: bool,
 
-    /// Continue the most recent conversation without showing the picker.
-    ///
-    /// Notes:
-    /// - Mutually exclusive with `--resume`.
-    /// - If no recorded sessions are found, this behaves like starting fresh.
-    /// - Equivalent to picking the newest item in the resume picker.
-    #[arg(
-        id = "continue",
-        long = "continue",
-        default_value_t = false,
-        conflicts_with = "resume",
-        hide = true
-    )]
-    pub r#continue: bool,
+    #[clap(skip)]
+    pub resume_last: bool,
+
+    /// Internal: resume a specific recorded session by id (UUID). Set by the
+    /// top-level `codex resume <SESSION_ID>` wrapper; not exposed as a public flag.
+    #[clap(skip)]
+    pub resume_session_id: Option<String>,
 
     /// Model the agent should use.
     #[arg(long, short = 'm')]