gate 0.5.0: provenance audit report

Added

• gate audit: a provenance report for a period (--since, --until), built from the target's first-parent
  history and the gate's session records. It lists every change merged through the gate (model, tool, spec, risk
  tier, approver, session), every automatic revert, every refused or unmerged attempt, and two kinds of finding:
  a commit that reached the target without the gate, and a change at or above approval_tier merged without
  Approved-By. --out writes the markdown report to a file, --json returns the same data, and --strict
  exits 2 when there is any finding, so CI can fail on a bypass.
• docs/provenance.md: the trailer format, version 1, with what it does and does not prove.
• SECURITY.md: how to report a vulnerability, and short answers to the usual security
  questionnaire, each pointing at the code or test that backs it.

CI: 61 tests green on Linux, macOS and Windows (Python 3.11 and 3.13), lint, and the two-host NFS test.