✨ Add recursive delete for non-empty directories

[anggrayudi]

Closes #640.

Adds opt-in recursive deletion of non-empty directories:

public expect suspend fun PlatformFile.delete(
    mustExist: Boolean = true,
    recursively: Boolean = false,
)

For filesystem paths, recursively = false keeps the existing non-empty-directory failure. With true, FileKit deletes the contents before removing the directory. Android document URIs continue to delegate deletion to the provider.

Links are inspected and unlinked before directory metadata is queried. This preserves outside targets and handles dangling and self-referencing links. Windows JVM uses reparse-point attributes so directory junctions are also recognized. Link removal bypasses kotlinx-io's target-existence check, which otherwise leaves dangling links behind.

Regression coverage

• Shared tests cover nested directories, ordinary files, missing paths, and the non-recursive default.
• JVM tests cover dangling links and outside-target preservation, plus a Windows-only junction test.
• Apple native tests cover dangling, cyclic, and outside-target links.
• Android host tests cover links at API 23 and 36. Their shadow supplies no-follow filesystem behavior because Robolectric's default lstat follows directory links.
• Updated the deletion documentation and AGENTS.md to keep local validation targeted; broad builds belong in CI because they overload the maintainer's Mac.

Validation

• JVM: 68 tests, 0 failures, 1 Windows-only skip on macOS. The dangling-link regressions failed before the fix and pass afterward.
• Android host: 62 tests, 0 failures.
• Linux and Windows native implementations compile from macOS.
• Repository-wide ktlint passed; changed Kotlin files were checked again after the final edits.
• Apple native runtime validation and Windows junction execution are pending CI. The local full build was stopped at the maintainer's request; it is not claimed as passing.

Manual native verification

1. In a disposable temporary directory, create outside/treasure.txt with known contents and doomed/link pointing to outside. On Windows, also repeat with a junction created using mklink /J.
2. Call PlatformFile(doomedPath).delete(recursively = true) in the target app. Verify doomed is gone and the sentinel's contents are unchanged.
3. Repeat with a dangling link and a self-referencing link inside doomed. The directory and links should all be removed.
4. Repeat with an ordinary non-empty directory and recursively = false; deletion should fail and its contents should remain.