chore: prepare 0.9.28 release

[wbugitlab1]

Summary

• prepare the 0.9.28 release metadata so the corrected package no longer ships Anthropic SDK packages as regular dependencies
• align plugin, integration, MCP package, deploy template, README, changelog, and export/import version surfaces
• add coverage for plugin surface version drift, including the OpenCode manifest

Addresses #171.

Verification

• corepack pnpm install --frozen-lockfile --ignore-scripts
• corepack pnpm exec vitest run test/export-import.test.ts test/quality-gates.test.ts test/plugin-surface-contract.test.ts test/codex-plugin.test.ts test/factory-plugin.test.ts test/deploy-version-contract.test.ts
• corepack pnpm run build
• corepack pnpm run lint
• npm pack --json --pack-destination /tmp
• inspected /tmp/agentmemory-agentmemory-0.9.28.tgz: no @anthropic-ai/* regular dependencies; @anthropic-ai/claude-agent-sdk remains an optional peer
• consumer package-lock smoke from the packed tarball: no node_modules/@anthropic-ai/sdk or node_modules/@anthropic-ai/claude-agent-sdk entries
• corepack pnpm --dir packages/mcp pack --dry-run --json
• corepack pnpm --dir packages/mcp pack --pack-destination /tmp
• git diff --check
• git diff --cached --check
• osv-scanner scan source .
• semgrep scan --config p/default --error --metrics=off .
• gitleaks protect --staged --redact