fix: agent instructions were not being included when ~/.codex/instructions.md was empty (#908)

I had seen issues where `codex-rs` would not always write files without
me pressuring it to do so, and between that and the report of
https://github.com/openai/codex/issues/900, I decided to look into this
further. I found two serious issues with agent instructions:

(1) We were only sending agent instructions on the first turn, but
looking at the TypeScript code, we should be sending them on every turn.

(2) There was a serious issue where the agent instructions were
frequently lost:

* The TypeScript CLI appears to keep writing `~/.codex/instructions.md`:
55142e3e6c/codex-cli/src/utils/config.ts (L586)
* If `instructions.md` is present, the Rust CLI uses the contents of it
INSTEAD OF the default prompt, even if `instructions.md` is empty:
55142e3e6c/codex-rs/core/src/config.rs (L202-L203)

The combination of these two things means that I have been using
`codex-rs` without these key instructions:
https://github.com/openai/codex/blob/main/codex-rs/core/prompt.md

Looking at the TypeScript code, it appears we should be concatenating
these three items every time (if they exist):

* `prompt.md`
* `~/.codex/instructions.md`
* nearest `AGENTS.md`

This PR fixes things so that:

* `Config.instructions` is `None` if `instructions.md` is empty
* `Payload.instructions` is now `&'a str` instead of `Option<&'a
String>` because we should always have _something_ to send
* `Prompt` now has a `get_full_instructions()` helper that returns a
`Cow<str>` that will always include the agent instructions first.

codex-rs/core/src/client.rs

@@ -166,9 +166,10 @@ impl ModelClient {
 
         debug!("tools_json: {}", serde_json::to_string_pretty(&tools_json)?);
 
+        let full_instructions = prompt.get_full_instructions();
         let payload = Payload {
             model: &self.model,
-            instructions: prompt.instructions.as_ref(),
+            instructions: &full_instructions,
             input: &prompt.input,
             tools: &tools_json,
             tool_choice: "auto",