Complete Slash, MCP Builder and Marketplace consolidation #512

Context

Issue #29 copied Slash, MCP Server Builder and Marketplace into SIN-Code, but the retirement/cutover remained incomplete.

Audit findings (2026-07-27)

• Standalone sin-mcp-server-builder: 120 tests pass.
• Standalone sin-marketplace: 97 test functions exist; logic passes, but the first audit run exposed a coverage-output cwd assumption.
• Standalone sin-slash: import fails before collection because class method list() shadows the built-in used by the evaluated annotation list[dict].
• SIN-Code's editable Python build fails with current Setuptools because an SPDX license expression is combined with the superseded license classifier.
• The three implementations are present under sin_code_bundle.tools.*, but old import namespaces (sin_slash, sin_mcp_server_builder, sin_marketplace) are not preserved.
• requirements-ecosystem.txt, ECOSYSTEM.md, lifecycle metadata, Go MCP registry and skill manager still treat Marketplace and MCP Builder as external repositories.
• The bundled script name is sin-marketplace-skill, while Go runtime discovery expects sin-marketplace.

Required work

• Fix PEP 639/Setuptools packaging metadata.
• Fix the Slash annotation/import failure in canonical code.
• Preserve old Python import namespaces as compatibility packages.
• Preserve legacy CLI entry points that have real consumers.
• Run the original standalone tests against the canonical compatibility surfaces.
• Register bundled Marketplace and MCP Builder MCP modules directly.
• Remove both repositories from external requirements/install-all/runtime registries.
• Update ECOSYSTEM, lifecycle docs and stale repository references.
• Back up all three standalone repositories and GitHub metadata.
• Add accurate deprecation READMEs pointing to SIN-Code.
• Archive repositories only after CI and consumer searches are green.

Non-goals

• No deletion of the standalone histories.
• No removal of compatibility entry points in this change.
• No unrelated feature redesign.