fix(renovate): group the toolbelt module with its Dockerfile ARG pin

[cplieger]

The toolbelt consumers pin github.com/cplieger/toolbelt/v2 twice on purpose:

• go.mod requires the runtime engine that re-verifies required-tools.txt before every catalog swap.
• ARG TOOLBELT_TOOLCATALOG_VERSION selects the build-time toolcatalog verify binary.

web-terminal-kiro's builder asserts the two are equal (toolbelt-pin-gate), so a half-bump does not drift silently — it fail-closes ci / docker, ci / docker-arm64, and the Trivy image scan until the second pin catches up.

Those two pins reach Renovate through different managers: gomod (swept into the Go dependencies group branch) and the generic Dockerfile-ARG custom.regex manager (its own per-dep branch). Nothing grouped them, so the v2.4.2 release produced half-PRs that could not be green and complete on their own:

• chore(deps): update module github.com/cplieger/toolbelt/v2 to v2.4.2 vibekit#827 — Dockerfile ARG only
• fix(deps): update module github.com/cplieger/toolbelt/v2 to v2.4.2 vibekit#828 — go.mod only
• fix(deps): update module github.com/cplieger/toolbelt/v2 to v2.4.2 web-terminal-kiro#666 — go.mod only, red on the pin gate

This adds a toolbelt group keyed on matchDepNames, the same cross-manager coupling model the kiro-cli and golang toolchain groups already use, and for the same reason: a pin pair that a build gate compares must never be splittable into two branches.

Notes on the shape:

• Placed after the gomod Go dependencies rule so its groupName wins the rule merge for toolbelt deps.
• separateMajorMinor left at the default — both pins are the same dep at the same version, so they always carry the same update type, and a toolbelt major stays behind the manual major gate.
• The /** entry keeps the group intact across a future Go semantic-import major (toolbelt/v3).

Validation: renovate-config-validator exits 0, and a minimatch check confirms the patterns match toolbelt, toolbelt/v2, and toolbelt/v3 without over-matching toolbelt-extra or any other cplieger/* module.