Merge bitcoin/bitcoin#34345: clang-format: use AngleBracket for main includes

0dafc0d83c3e4c48c0ab2efad4b160d1b8c6d252 clang-format: use AngleBracket for main includes (stickies-v)

Pull request description:

  This project uses angle brackets instead of quotes for project-specific headers. Setting [`MainIncludeChar`](https://clang.llvm.org/docs/ClangFormatStyleOptions.html#mainincludechar) enables `clang-format` to automatically detect the main header, so it can be kept as the top group of includes.

  For example, without this change, `clang-format` would demote `<signet.h>` from being the main header in `src/signet.cpp`. With this change, the order is preserved.

  On 5e49f5d63c74512c8f46fe7de7deb0341f13244a:
  ```
  % clang-format src/signet.cpp | head -n 15
  // Copyright (c) 2019-present The Bitcoin Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.

  #include <consensus/merkle.h>
  #include <consensus/params.h>
  #include <consensus/validation.h>
  #include <logging.h>
  #include <primitives/block.h>
  #include <primitives/transaction.h>
  #include <script/interpreter.h>
  #include <script/script.h>
  #include <signet.h>
  #include <streams.h>
  #include <uint256.h>

  ```

  With this PR:
  ```
  % clang-format src/signet.cpp | head -n 10
  // Copyright (c) 2019-present The Bitcoin Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.

  #include <signet.h>

  #include <consensus/merkle.h>
  #include <consensus/params.h>
  #include <consensus/validation.h>
  #include <logging.h>

  ```

  Note: `AngleBracket` `requires clang-format 19`, and will cause older versions (including our current minimum llvm version `17`) to fail

ACKs for top commit:
  maflcko:
    review ACK 0dafc0d83c3e4c48c0ab2efad4b160d1b8c6d252
  sedited:
    Nice, ACK 0dafc0d83c3e4c48c0ab2efad4b160d1b8c6d252
  hebasto:
    ACK 0dafc0d83c3e4c48c0ab2efad4b160d1b8c6d252, tested on Ubuntu 25.10.

Tree-SHA512: c0876f505ec188f76e435af0731c411c66266b83e4c08528d0637263abcd84b3968ee6fbfa72630192f1a0cd2728af873d3d6c32f93ab8b228222fad16f232be

src/.clang-format

@@ -142,6 +142,7 @@ LambdaBodyIndentation: Signature
 LineEnding:      DeriveLF
 MacroBlockBegin: ''
 MacroBlockEnd:   ''
+MainIncludeChar: AngleBracket
 MaxEmptyLinesToKeep: 2
 NamespaceIndentation: None
 ObjCBinPackProtocolList: Auto