fix(exception): carry PHP's $previous through the ported throw sites

Composer hands the exception it caught to the one it throws in its place,
so `getPrevious()` reaches the cause and Application's renderer prints the
whole chain. Every ported site dropped it, because the flat exception
structs had nowhere to put one. `AnyThrowable::into_previous` turns the
caught error into that argument, and the 13 sites now pass it.

`getCode()` came along for the ride at the four sites that derive the new
exception's code from the caught one (PharArchiver, ArrayLoader x2), and
ComposerRepository's message now names the caught exception's class
instead of the literal "Exception".

GitHubDriver::attemptCloneFallback took the previous exception's message
and appended it to its own, which no `\RuntimeException('Fallback to git
driver disabled')` in Composer ever says; it now chains it instead.

Git::syncMirror restores what PHP's `finally` does to an exception in
flight: the `git remote set-url` that scrubs credentials back out of the
URL runs in a `finally`, and when it fails PHP propagates *its* exception
over the one already leaving, chaining the displaced one as previous. The
port discarded the finally's result, so a failure to scrub the URL was
reported as a successful mirror sync. `AnyThrowable::set_previous`
models the engine-level chaining.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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