feat(cli): expose MCP HTTP mode (#1121)

## Description
Adds `codemem mcp http` as the CLI entrypoint for the Streamable HTTP MCP server while preserving `codemem mcp` as the stdio default.

The new subcommand forwards `--db-path`/`--db`, `--host`, `--port`, and `--unsafe-public` to `@codemem/mcp/http`. Host and port are left unset when omitted so environment/default resolution stays in the MCP HTTP package. README documentation covers the command, env vars, loopback default, and unsafe-public caveat.

## Type of Change

- [x] 🚀 Feature (new functionality)
- [ ] 🐛 Bug fix (fixes an issue)
- [x] 📚 Documentation (docs-only change)
- [ ] 🔧 Maintenance (refactor, chore, CI, etc.)
- [x] 🧪 Testing (test-only changes)

## Testing

- [x] Relevant checks pass locally (`pnpm run tsc`, `pnpm run lint`, `pnpm run test`)
  - `pnpm --filter codemem typecheck`
  - `pnpm --filter @codemem/mcp typecheck`
  - `pnpm exec vitest run packages/cli/src/commands/mcp.test.ts packages/mcp-server/src/http.test.ts`
- [x] Added/updated tests for changes
- [x] Manually verified changes work as expected

## Checklist

- [x] Code follows project style (`pnpm run lint` passes for touched files)
- [x] Self-review completed
- [x] Documentation updated (if needed)
- [x] No new warnings introduced