fix: PathBuf -> AbsolutePathBuf in ConfigToml struct (#8205)

We should not have any `PathBuf` fields in `ConfigToml` or any of the
transitive structs we include, as we should use `AbsolutePathBuf`
instead so that we do not have to keep track of the file from which
`ConfigToml` was loaded such that we need it to resolve relative paths
later when the values of `ConfigToml` are used.

I only found two instances of this: `experimental_instructions_file` and
`experimental_compact_prompt_file`. Incidentally, when these were
specified as relative paths, they were resolved against `cwd` rather
than `config.toml`'s parent, which seems wrong to me. I changed the
behavior so they are resolved against the parent folder of the
`config.toml` being parsed, which we get "for free" due to the
introduction of `AbsolutePathBufGuard ` in
https://github.com/openai/codex/pull/7796.

While it is not great to change the behavior of a released feature,
these fields are prefixed with `experimental_`, which I interpret to
mean we have the liberty to change the contract.

For reference:

- `experimental_instructions_file` was introduced in
https://github.com/openai/codex/pull/1803
- `experimental_compact_prompt_file` was introduced in
https://github.com/openai/codex/pull/5959

docs/example-config.md

@@ -311,7 +311,7 @@ experimental_use_freeform_apply_patch = false
 # model_reasoning_summary = "auto"
 # model_verbosity = "medium"
 # chatgpt_base_url = "https://chatgpt.com/backend-api/"
-# experimental_compact_prompt_file = "compact_prompt.txt"
+# experimental_compact_prompt_file = "./compact_prompt.txt"
 # include_apply_patch_tool = false
 # experimental_use_freeform_apply_patch = false
 # tools_web_search = false