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

docs/getting-started.md

@@ -10,19 +10,24 @@
 
 Key flags: `--model/-m`, `--ask-for-approval/-a`.
 
-<!--
-Resume options:
+### Resuming interactive sessions
 
-- `--resume`: open an interactive picker of recent sessions (shows a preview of the first real user message). Conflicts with `--continue`.
-- `--continue`: resume the most recent session without showing the picker (falls back to starting fresh if none exist). Conflicts with `--resume`.
+- Run `codex resume` to display the session picker UI
+- Resume most recent: `codex resume --last`
+- Resume by id: `codex resume <SESSION_ID>` (You can get session ids from /status or `~/.codex/sessions/`)
 
 Examples:
 
 ```shell
-codex --resume
-codex --continue
+# Open a picker of recent sessions
+codex resume
+
+# Resume the most recent session
+codex resume --last
+
+# Resume a specific session by id
+codex resume 7f9f9a2e-1b3c-4c7a-9b0e-123456789abc
 ```
--->
 
 ### Running with a prompt as input