feat(php-shim): render human-facing timestamps in the local timezone

Split date() into date_utc() and date_local(), the latter resolving the
system's local timezone through the tzfile crate ($TZ, then
/etc/localtime, falling back to UTC when neither is readable). The
timestamps Composer renders for humans -- the GitHub OAuth token note,
the GitHub API rate limit reset time, the Perforce client spec fields and
the "today" check of the show command -- now go through date_local().

PHP resolves its default timezone from the date.timezone ini setting,
which Shirabe does not read, so date_default_timezone_get/set have no
input left to model and are dropped from the shim and its callers. The
resulting difference is recorded in docs/known-incompatibilities.md.

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

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