harness 0.12.1

Changed

• Update the embedded confinement boundary to Substrate 3fafeae6 (release 0.7.7), which leaves
  Git metadata untouched during host startup, Git-only observations and workspace destruction when
  no Git sources are configured. Both b10x-substrate-host and b10x-substrate-wire keep an exact
  =0.7.7 version requirement beside the revision, so a resolver cannot substitute a later
  contract; the development wire bundle remains 0.16.0.
• Update the outbound MCP client crates and both testkit declarations to bf7f9415 (release
  0.1.2). The release carries documentation-surface and publication changes only; no client API,
  transport or credential behaviour changed.
• Consume @beyond10x/docs-system at revision 86cd6c6e (release 0.7.0) in
  website/package.json and website/package-lock.json. The documentation bundle and Pages
  delivery workflows keep their own catalog-owned pins and are unchanged.

The full cargo xtask gate is green against the updated Cargo.lock.