feat(plugins): add persistent private filesystem directories (#2051)

Closes #1947

Adds API 1.3 `fs.plugin-dir`: a persistent owner-only directory per plugin (0700 directories / 0600 files on Unix, owner-only ACL on Windows), separate from the packaged code, storage and secrets. The plugin reads and writes under it without fs.* grants, can open files there in the editor and pass their paths to approved programs; editor saves keep the protection, restart keeps the content, uninstall removes it. Traversal and symlink escapes into other plugins' directories are rejected (real-Wasm regressions).

Co-authored-by: arnaud-ritti <[email protected]>

plugins/official/string-tools/dist/plugin.wasm.sources

@@ -1 +1 @@
-730d2bc7b15d2598840ec8668555f89cdf77b800c2ce21df7601880c57be6b2b
+428dd3b2ebc9bc248775e385fc431f0621e564533ed54f70d12bf5db950627cb