AXI-85: Fix rich link URL scheme safety

[Codename-11]

Summary

• add shared URL-safety helper for renderable markdown hrefs and external link attachments
• keep markdown https://... and internal /w/... links working while rendering unsafe schemes inert
• enforce http(s)-only link attachments across tRPC, MCP, storage, and guard legacy link-attachment open/preview surfaces
• add regression coverage for helper, markdown renderer, tRPC attachLink, and MCP attachLink

Verification

• env -u OPENAI_API_KEY pnpm exec vitest run tests/unit/url-safety.test.ts tests/unit/markdown-url-safety.test.ts src/server/services/__tests__/mcp.test.ts src/server/routers/__tests__/attachment.test.ts --reporter=verbose --testNamePattern "url safety|MarkdownWithAttachments URL safety|attachments.attachLink accepts only http\(s\) external URLs|rejects non-http"
• pnpm typecheck
• pnpm lint
• env -u OPENAI_API_KEY pnpm test (946 passed, 1 skipped)

Fixes AXI-85.