d084bc88be doc: clarify IWYU workflow (Lőrinc)
7c7cec4567 ci: update IWYU patch reference (Lőrinc)
Pull request description:
### Problem
This was prompted by https://github.com/bitcoin/bitcoin/pull/34435#discussion_r3123255248, where it was not clear to me how (and where) exceptional IWYU cases should be documented.
### Fix
This PR documents the IWYU CI wrapper as the reproducible local entrypoint.
The developer notes now recommend reducing suspected IWYU false positives to a minimal upstream reproducer, treat `IWYU pragma` as a narrow workarounds, and ask for nearby rationale comments on non-obvious IWYU pragma use. An example comment was also added.
The IWYU patch comment is also updated to point at the current `clang_22` include picker reference.
### Reproducer
Create a dummy commit on top that adds an unused include, then run the command from the developer notes.
Without the dummy commit, the command should pass.
<details><summary>IWYU demo commit</summary>
```diff
diff --git a/src/kernel/bitcoinkernel.cpp b/src/kernel/bitcoinkernel.cpp
--- a/src/kernel/bitcoinkernel.cpp(revision c92b329e7b7d49476b5977d26c24d7c4982c6024)
+++ b/src/kernel/bitcoinkernel.cpp(revision ad2c5ba2ba69156e77061c1e6c098b725c28f322)
@@ -43,6 +43,7 @@
#include <functional>
#include <list>
#include <memory>
+#include <vector>
#include <span>
#include <stdexcept>
#include <string>
```
</details>
> [!NOTE]
> After repeated failing runs, `docker container rm -f ci_native_iwyu` may be needed because the local CI wrapper can leave the detached container running when the inner test command fails.
ACKs for top commit:
hebasto:
ACK d084bc88be.
sedited:
ACK d084bc88be
Tree-SHA512: 0aac42d468a1fdfa9f4a3856372e05fb43ec9f0973aeb3a4194fff948fc61e8e72e3b280cde10e74b8da88b6cff93962b3a7f7390eb042113ef92aa6b51d6d8f
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