Add remote --cd forwarding for app-server sessions (#16700)

Addresses #16124

Problem: `codex --remote --cd <path>` canonicalized the path locally and
then omitted it from remote thread lifecycle requests, so remote-only
working directories failed or were ignored.

Solution: Keep remote startup on the local cwd, forward explicit `--cd`
values verbatim to `thread/start`, `thread/resume`, and `thread/fork`,
and cover the behavior with `codex-tui` tests.

Testing: I manually tested `--remote --cd` with both absolute and
relative paths and validated correct behavior.


---

Update based on code review feedback:

Problem: Remote `--cd` was forwarded to `thread/resume` and
`thread/fork`, but not to `thread/list` lookups, so `--resume --last`
and picker flows could select a session from the wrong cwd; relative cwd
filters also failed against stored absolute paths.

Solution: Apply explicit remote `--cd` to `thread/list` lookups for
`--last` and picker flows, normalize relative cwd filters on the
app-server before exact matching, and document/test the behavior.

codex-rs/app-server/README.md

@@ -273,7 +273,7 @@ Experimental API: `thread/start`, `thread/resume`, and `thread/fork` accept `per
 - `modelProviders` — restrict results to specific providers; unset, null, or an empty array will include all providers.
 - `sourceKinds` — restrict results to specific sources; omit or pass `[]` for interactive sessions only (`cli`, `vscode`).
 - `archived` — when `true`, list archived threads only. When `false` or `null`, list non-archived threads (default).
-- `cwd` — restrict results to threads whose session cwd exactly matches this path.
+- `cwd` — restrict results to threads whose session cwd exactly matches this path. Relative paths are resolved against the app-server process cwd before matching.
 - `searchTerm` — restrict results to threads whose extracted title contains this substring (case-sensitive).
 - Responses include `agentNickname` and `agentRole` for AgentControl-spawned thread sub-agents when available.