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/advanced.md

@@ -12,6 +12,40 @@ Run Codex head-less in pipelines. Example GitHub Action step:
     codex exec --full-auto "update CHANGELOG for next release"
 ```
 
+### Resuming non-interactive sessions
+
+You can resume a previous headless run to continue the same conversation context and append to the same rollout file.
+
+Interactive TUI equivalent:
+
+```shell
+codex resume             # picker
+codex resume --last      # most recent
+codex resume <SESSION_ID>
+```
+
+Compatibility:
+
+- Latest source builds include `codex exec resume` (examples below).
+- Current released CLI may not include this yet. If `codex exec --help` shows no `resume`, use the workaround in the next subsection.
+
+```shell
+# Resume the most recent recorded session and run with a new prompt (source builds)
+codex exec "ship a release draft changelog" resume --last
+
+# Alternatively, pass the prompt via stdin (source builds)
+# Note: omit the trailing '-' to avoid it being parsed as a SESSION_ID
+echo "ship a release draft changelog" | codex exec resume --last
+
+# Or resume a specific session by id (UUID) (source builds)
+codex exec resume 7f9f9a2e-1b3c-4c7a-9b0e-123456789abc "continue the task"
+```
+
+Notes:
+
+- When using `--last`, Codex picks the newest recorded session; if none exist, it behaves like starting fresh.
+- Resuming appends new events to the existing session file and maintains the same conversation id.
+
 ## Tracing / verbose logging
 
 Because Codex is written in Rust, it honors the `RUST_LOG` environment variable to configure its logging behavior.