fix: reconcile regression gate test scope with documented full-suite policy

[dennisonbertram]

Summary

Reconciles the regression test gate scope with the documented full-suite policy in docs/runbooks/testing.md.

Problem

• scripts/test-regression.sh used hardcoded ./internal/... ./cmd/... as its test scope
• docs/runbooks/testing.md documented the regression gate as enforcing go test ./...
• The mismatch meant operators and agents couldn't trust the docs/script contract

Solution

• scripts/test-regression.sh: Changed PKG_PATTERNS default from ./internal/... ./cmd/... to ./..., derived via go list ${PKG_PATTERNS}. Kept the variable overridable for flexibility.
• Added COVER_PKG_PATTERNS (defaults to ./internal/... ./cmd/...) so the coverage gate remains practical on core packages while tests run against the full module.
• Added drift tests in internal/quality/repostructure/regression_scope_test.go that:
  • Assert the script defaults PKG_PATTERNS to ./...
  • Assert the script has a separate coverage scope for core packages
  • Assert the docs reference go test ./...
  • Assert script and docs agree on scope (drift guard)

Test Results

• go test ./internal/quality/repostructure -count=1 -- all 6 tests PASS (including 4 new drift tests)
• go test ./cmd/coveragegate -count=1 -- all 5 tests PASS
• go test ./internal/... ./cmd/... -count=1 -- all packages PASS
• go test ./... -count=1 -- all packages PASS
• ./scripts/test-regression.sh -- go test and go test -race steps PASS on full ./... scope; coverage gate reports pre-existing zero-coverage functions (not caused by this change).

Design Decisions

• Test scope widened to ./... to match documented full-suite policy
• Coverage scope kept at ./internal/... ./cmd/... to keep the 80% threshold practical
• Both scopes overridable via env vars for CI flexibility
• Drift tests prevent silent regression of the docs/script contract

Closes #568

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