feat(verification): add GoLang and Markdown Docker verification targets; add version pinning

[apoorv7g]

This adds end-to-end Go and Markdown verification to the codegen pipeline: GitHub Actions matrix support, npm docker-run script wiring, new docker targets/images/entrypoints, and local mocha suites that generate each target and run the corresponding verifier (go build / markdownlint).

For Go, it also updates verification case metadata and corpus manifest with explicit golang skip reasons for known cross-namespace import path issues emitted by GoLangVisitor, keeping failures intentional and documented.

Changes

GoLang

• Add verification/docker/golang/Dockerfile and entrypoint.sh: layers the golang:1.25-alpine toolchain onto the shared concerto-verify-base image, generates Go via concerto compile --target GoLang, fans generated files out into per-package subdirectories (Go requires one package per directory, unlike the flat output the visitor emits), and runs go build ./...
• Register golang in scripts/verification/docker-run.js TARGETS, .github/workflows/verify-codegen.yml matrix, and verification/docker/targets.json
• Add verify:docker:golang npm script to package.json, matching the existing per-target script pattern
• Add test/verification/golang.compile.test.js: generates Go from each verification case via GoLangVisitor, mirrors the same per-package fan-out fix, writes a synthetic go.mod, and runs go build ./..., skipping gracefully when the go toolchain isn't installed
• Add golang skip reasons to all 7 cases in test/verification/cases.js and to the metamodel case in verification/corpus/manifest.json, documenting a pre-existing GoLangVisitor bug (bare package-name imports instead of Go module-relative import paths) that breaks go build for any model with cross-namespace imports (e.g. anything referencing concerto.decorator)

Markdown

• Add verification/docker/markdown/Dockerfile and entrypoint.sh: installs markdownlint-cli@0.44.0 on the shared concerto-verify-base image, generates Markdown via concerto compile --target Markdown, and runs markdownlint against each produced .md
• Add verification/docker/markdown/markdownlint.json: shared lint config that disables stylistic rules commonly tripped by generated docs (line length, trailing whitespace, blank-line spacing around lists/fences, etc.) while keeping substantive markdownlint defaults
• Register markdown in scripts/verification/docker-run.js TARGETS, .github/workflows/verify-codegen.yml matrix, and verification/docker/targets.json
• Add verify:docker:markdown npm script to package.json, and add markdownlint-cli as a devDependency for local runs
• Add test/verification/markdown.validate.test.js: generates Markdown from each verification case via MarkdownVisitor and lints with the same markdownlint.json config (overridable via MARKDOWNLINT_CONFIG)
• Document the Markdown (and Go) verification flow in verification/VERIFICATION.md

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname