fix: add ZEND_TRACK_ARENA_ALLOC on ext build option, clean-build-cache command

[zeriyoshi]

No description provided.

[Copilot]

Pull request overview

Adds -DZEND_TRACK_ARENA_ALLOC to extension compilation flags for specialized test modes (debug/gcov/valgrind/msan/asan/ubsan) so the extension's macro usage matches the PHP runtime it links against, and introduces a new clean-build-cache command to remove built PHP runtimes and their cached copies.

Changes:

• Conditionally set PSKEL_TRACK_ARENA_ALLOC in cmd_test and append -DZEND_TRACK_ARENA_ALLOC to CFLAGS/CPPFLAGS for specialized builds.
• Add cmd_clean_build_cache plus helpers remove_path_if_exists and remove_built_php_runtime, wired into usage and dispatch.
• Document the new clean-build-cache command in README.md and README_ja.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pskel.sh	Adds arena-alloc tracking flag handling in cmd_test and a new clean-build-cache command with helpers.
README.md	Documents the new clean-build-cache command and realigns the command list.
README_ja.md	Mirrors the English doc update for clean-build-cache.

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