v0.16.0-preview.15

Fix Windows and Linux host write-permission diagnostics. New installations retain trusted-host + all command access, while ACCESS_DENIED now distinguishes operating-system account permissions from WebCodex policy and directs users to webcodex access check.

Validation: 1081 tests, 1077 passed, 0 failed, 4 platform-permission skips; release package and installer suites passed.